Fix crash in VisualBasic.Binder.MemberLookup.AddLookupSymbolsInfoInTypeParameter when it is called with Cref TypeParameter. (#21586)

Fixes https://devdiv.visualstudio.com/DefaultCollection/DevDiv/_workitems?id=410932.

The fix mimics the way C# compiler handles Cref TypeParameters in the similar API.

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

are marked not usable in C#7

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

* Compare lines instead of using AssertXml when validating PDBs.

AssertXml doesn't validate ordering.

* Reorder expected PDB XML nodes to match actual order

* Update to the latest Microsoft.DiaSymReader.* packages.

* Enable embedding sources to Windows PDBs

Merged on behalf of @svick 
Thanks for the contribution!

* Clean up Concord exception utilities

* Avoid dependency on IL stream in EnC local slot mapping

* Fix lambda EnC

* Tagging VB/C# Ioperation tests

* Fix the Wrong Tagging

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).

This is part of the effort to move as much logic from LambdaRewriter
into Analysis as possible. The more logic there is in the Rewriter, the
more difficult it is to find calculation problems until the last
possible moment, and it's also very difficult to calculate useful
information for later analysis passes, like the final debug and closure
IDs for each environment.