This unifies our API naming convention. When standard is in the name
then we are using the netstandard2.0 API set and when it is not then it
is the simple desktop mscorlib46.

Both variants are necessary for CompileAndVerify style functions because
many of our tests that use it can only run on desktop or simply need to
have some of the netstandard2.0 API set missing.

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems?id=481125

Fixes #21768
Fixes #22027

Fixes #21811
Fixes #21645
Fixes #21543

In order for EnC and other mechanisms to work we have to add synthesized
members to a list in the CommonPEModuleBuilder for a compilation (these
synthesized members are then queried as part of compilation stages).
If those members are struct fields, we don't add them to the struct
definition, only to the list of synthesized members. This works for
emit, since we explicitly emit everything in the synthesized list, but
it doesn't work for any compiler pass that examines the members of the
struct for semantically meaningful reasons.

This is the case for the CaptureWalker for async and iterator
expressions. The walker checks the members of structs when a field of a
struct is assigned to see if the struct has been assigned piecewise
(each of its members has been assigned individually). If so, it will
mark the entire struct as assigned. By not including synthesized fields
as proper members of the struct type, the assignment pass believes that
many fields have been assigned that have not, and thus marks the full
struct as assigned, leading to losing track of variables captured across
await/yield statements.

This PR fixes the problem by adding the fields to the
SynthesizedContainer, but exlcuding them from emit, since we should
continue to use the emit mechanism used for CommonPEModuleBuilder.

Fixes #21409

Perform synthesis of closure methods in an early pass before visitation,
meaning we no longer need to do a second visitation of the tree to lower
local functions.

The baselines have been changed because we now do closure id and
synthessis in order of closure visitation, rather than bound node
visitation. Closure visitation visits all the closures in a given scope,
then recurs into nested scopes, while BoundNode visitation treats
closures as scopes themselves, so nested closures are visited before
closures declared in the same scope.

Fixes https://github.com/dotnet/roslyn/projects/26#card-3753331

In the introduction of the new 'this' optimization routine,
one of the things which was changed was to treat 'this' more
like a formal parameter of the method, as opposed to a variable
living in an implicit, higher scope. This has some advantages
in simplicity for analysis, but created a problem when it came
to proxies. The current analysis builds the proxy list by walking
the tree and finding all captured variables and adding them
to the proxy dictionary keyed by the original variable symbol.
For instance, if a local variable is captured to a field, during
rewriting it will be added to the proxy list as (original symbol,
hoisted field). Since most symbols are only ever captured to a
single replacement field, this usually works fine -- all proxies
can exist side-by-side in the proxy list since there is no intersection.

However, this is not true for captured environment pointers. When
a new environment is introduced, a local will be created to point
to that environment. That local may itself be captured by nested
variables, creating a linked list from nested scopes to parent scope.
Most notably, *multiple* nested environments may capture the *same*
environment pointer in *different* hoisted fields. This means that
the proxies dictionary cannot hold all mappings at once, since the
mapping for a given captured environment pointer will depend on the
current scope.

The current code actually accounts for this already by adding a
captured environment pointer to the proxy list on a nested scope's
introduction, and removing it upon leaving that scope.

By changing 'this' to be treated like a formal parameter, I
circumvented this logic, introducing a bug. When two scopes tried
to capture the 'this' pointer, the compiler crashed due to trying
to add two mappings to the same key.

This change fixes this problem by treating the 'this' parameter
like an environment pointer for the purposes of capturing and
hoisting. It's possible that we want to treat it like a formal
parameter, but if so it's probably better to treat all captured
environment pointers the same way and introduce a scope-aware
notion of proxies, rather than having a global dictionary.

Fixes #21506

Currently, the lambda rewriter has an early optimization pass in
analysis that tries to find all local functions that only capture 'this'
and remove references to local functions that do the same. There are two
problems with this approach:

    1) Generally, removing information from the tree is a bad idea
    because it hurts further analysis passes that may have needed that
    information.

    2) The optimization strategy itself is very tricky and has a number
    of complex corner cases. This has lead to bugs, for example #19033.

This PR deletes the current method and adds a new optimization routine
at the end of the analysis, operating on assigned scopes and
environments rather than removing captured variable analysis. The new
optimization is as follows: if we end up with an environment containing
only 'this', the environment can be removed, all containing methods can
be moved to the top-level type, and all environments which capture the
'this' environment can instead directly capture the 'this' parameter.
This produces almost the same results as the previous optimization, but
is easier to validate as an algebraic equivalence.

The baseline changes come from the new optimization being less aggressive 
about moving functions which only capture 'this' to the top level. This
appears to be a wash -- some codegen gets slightly better, some gets
slightly worse.

Fixes #19033
Fixes #20577

The frame creation pass now considers scope of captured variable
introduction instead of closure creation, so the ordering can change if
two or more nested functions access variables 'outside-in', e.g.

```csharp
{
    int x = 0;
    {
        int y = 0;
        int L1() => y;

        {
             int z = 0;
             int L2() => x;
        }
    }
}
```

If we visit closures in-order in the previous example, we create frames
for L1, then L2. If we visit captured variables scopes, however, we
visit `x` (captured by L2), then `y` (captued by L1).

* Rename Foo to Goo

* Rename ifoo to igoo

* Add back BOMs

Swaps out all uses of CapturedVariablesByLambda for functions on the
Scope tree.

The reason the baseline changed is that the order of enumerating closures changed. I briefly looked into matching the behavior, but the problem is that the closure order in the old visitors is determined by the visitation of captured variables, not visitation of the closures themselves. More specifically, an item is only added to the CapturedVariablesByLambda dictionary when ReferenceVariable encounters a captured variable.

In contrast, the visitation of the Scope tree looks at closures in an in-order traversal of the tree, stopping at the first introduced closure, not necessarily the closure which first captured a variable.

One obvious consequence of this changing is that the old ordering for a series of nested lambdas was effectively post-order -- the interior closures would be added to the visitation first, since they would be added from the perspective of the captured variable looking up, instead of the visitor looking down. This particular case is responsible for all the baseline changes that I have seen.

When I looked into replicating this traversal order I found it complicated and pretty counter-intuitive. Since the baseline change was so low in emitted IL, I thought it better to keep a very natural in-order tree traversal and just rebaseline a few tests.

Fixes https://github.com/dotnet/roslyn/projects/26#card-3753318

When lowering local functions that capture struct frames, the frames are
added as ref parameters to the lowered method. Normally, when the local
function captures variables from multiple levels up the intermediate
closures are also rewritten to take the ref parameters. However, if one
of the intermediate closures captures no variables, the closure
conversion code can skip adding ref environments to that closure. This
can cause the inner-most closure to no longer have access to all of the
ref structs it expects, which crashes the compiler.

Fixes #18814, #18918

enabled one test for a fixed bug.

Lambda rewriting currently allows frames to be structs in the instance
where

The variables in the frame are not captured by any closures
converted to delegates.

A closure nested inside the struct closure captures a separate
variable that does require a class frame.

This poses a problem when the "outer" closure is in the same frame as
the variable that requires a class closure. The problem is that we treat
scopes and frames as essentially the same thing -- if any scope is
perceived to require a particular environment type, all captured
variables in that scope will be captured on that frame and all closures
will be lowered onto that frame.

This creates a conflict between class-based environment capturing, where
we want to capture the frame pointer as a field, and struct-based frame
capturing, where we want to add arguments as ref parameters. Doing both
results in breaking the delegate contract (no signature rewriting) or
losing required struct arguments to intermediate closures.

To elaborate on the problem here, let's consider the example:

```csharp
using System;
class C
{
    public void M()
    {
        int x = 0;
        {
            int y= 0;
            void Local()
            {
                if (x == 0)
                {
                    Action a = () => y++;
                    a();
                }
            }
            Local();
        }
    }
}
```

The current problem in the compiler is in the code now in GetStructClosures. The previous implementation only built struct closure if the closure was being lowered onto a struct frame.

However, in the example, y is being captured by a lambda, meaning that y must be in a class frame. Since Local lives in the same scope it also lives in the same frame and contains the lambda capturing y, meaning that it must live in a class frame as well. Of course, it is not converted to a delegate, nor does it capture any variables that are captured by closures converted to delegates, so its signature is free to be rewritten to take variables which are not captured by closures converted to delegates as struct environment parameters.

Here is the expected lowering for the previous example:

```csharp
void M()
{
    var env1 = new Env1();
    env1.x = 0;
    var env2 = new Env2();
    env2.y = 0;
    env2.<>_L(ref env1);
}

struct Env1
{
    public int x;
}

class Env2
{
    public int y;
    public void <>_L(ref Env1 env1)
    {
        if (env1.x == 0)
        {
            var env3 = new Env3();
            env3.env2 = this;
            Action a = env3.<>_anon;
            a();
        }
    }
}

class Env3
{
     Env2 env2;
     public void <>_anon() => this.env2.y++;
}
```

The problem comes when calculating the struct frames needed to be passed to Local. In the current implementation, unless the "container" is a struct, no struct frames are added. If we fix that to check for struct frames that are required as parent frames, regardless of the type of container we run into another problem.

In this case, the lambda also includes struct frames in its parent "context", but adding any struct frames to a closure that is converted to a delegate is incorrect.

The previous problem can be solved by a check to see if the closure can take new ref parameters. This is true for all closures except for ones marked async or iterators or ones converted to delegates. This works because of something we already know from the analysis: if we assume that struct environments are only created when the their capturing closure can take ref parameters, we can assume that any struct environments in scope are "safe" to use, even if those frames are in the "parent context" of the closure.

Fixes #16895

Make local function default parameter value binding lazy

Current strict binding can cause circularity problems when local
functions are referenced. This change causes local functions to use lazy
default parameter binding, similar to methods, and then forces their
construction when diagnostics are requested for the local function.

This also requires a mechanism for recording declaration diagnostics
outside of adding to the compilation's DeclarationDiagnostics. A new
type, DeclarationDiagnosticStore is introduced as an abstraction to
store declaration diagnostics on either the compilation or in a local
DiagnosticBag, depending on the needs of the symbol storing diagnostics.

Fixes #16451, #17293

Fixes #16352

Attributes will not be permitted on type parameters or parameters of
local functions. This matches the existing behavior for the return type
and symbol itself, neither of which allow attributes to be applied to
them.

Fixes #1640

* Add regression test for #16399

Closes #16399

Fixes #16038

See #15751

There were two root causes here:

    1) The rewriter was treating the difference between the symbol and
       `symbol.ConstructedFrom` as whether or not there were any type
       parameters remaining that may need substitution. This is invalid
       for recursive local functions.

    2) The local function reference rewriter was including type
       parameters from the containing type in the list of parameters to
       substitute. This should happen iff the containing type is a
       lambda frame used to capture variables, which is not always the
       case for local functions (although it is always the case for
       lambdas).

Fixes #15751

Local functions have already implemented their own frame management,
including frame pointer "capturing" (which is implemented by passing a
list of frames to the local function by-ref, rather than traditional
lambda capturing which is implemented by keeping a linked list of frame
pointers to parent frames).

PR #14736 wired up local function lowering to the lambda frame pointer
machinery, which is necessary for when a local function is captured by a
lambda or converted to a delegate, but in the process it diverted
control flow for pure local function capturing to the frame pointer
machinery, which can't handle the structure of local function struct
frame pointers.

This PR resolves the issue by restoring the control flow for local
functions capturing other local functions with by-ref struct frames.

Fixes #15599

When local functions are declared in unsafe regions (unsafe statement,
unsafe member, unsafe type), their bodies should be considered unsafe
regions, even if unsafe is not a declaration modifier on the local
function.

Fixes #13172

Prior to C# 7 we tried to avoid giving duplicate errors
when the user used a variable before it was declared (which
is both a use-before-declared and a use-before-assigned error),
but the line here is blurred with local functions, which can
use variables before they are declared in subtle ways that can't
be detected based on declaration order in syntax.

It may be possible to provide the earlier behavior, but the
current optimization is definitely insufficient and hides
legitimate errors in user programs.

Fixes #15298
Fixes #15322

(cherry picked from commit 1c8372ce)