The semantic model often creates ReducedExtensionMethods for the public API,
but the creation of ReducedExtensionMethod does constraint checking, which
requires a compilation, which isn't always available. This change removes constraint
checking from all creation of ReducedExtensionMethod and adds asserts that we
always have a compilation during constraint checking.

Fixes #37021

Report a better error for an attempt to access protected member of object class within an interface.

Before this change, the following confusing error was reported for all error locations in the added test scenario:
CS1540 Cannot access protected member 'object.MemberwiseClone()' via a qualifier of type 'object'; the qualifier must be of type 'I1' (or derived from it)

e.g. `ReturnType`, `ElementType`, etc.

- Rename `TypeSymbolWithAnnotations` to `TypeWithAnnotations`
- Rename its type field from `TypeSymbol` to `Type`
- Fields of Symbols that are of type `TypeWithAnnotations` that are currently named `Type` would be renamed `TypeWithAnnotations`
Fixes #33736

This includes tests for

- Language version checks for ref in for & foreach
- Additional tests around unary plus and ref assignment expressions
- Tests for ?., IDisposable, and ref assignment expressions
- Additional semantic model tests
- Additional IOperation tests

These are the blocking issues identified for the test plan https://github.com/dotnet/roslyn/issues/22466

- Implement improved overload candidates, aka "Bestest Betternes".
- Refine candidate set based on static/instance receiver, or static context
- Refine candidate methods whose type parameter constraints are violated.
- For a method group conversion, remove methods with the wrong return type or ref mode.
- Implement improved diagnostics for some cases of method group conversion failure

Implements https://github.com/dotnet/csharplang/issues/98
Fixes #24675

This changes the reference assemblies we use for the CoreClr compiler
compilation to be Net Standard 2.0 RTM. Previously they were a beta
version.

This did mean a number of tests had to be moved to be desktop specific
again. This is primarily due to the presence of ValueTuple as a default
type in Net Standard 2.0. Tests that relied on ValueTuple not being
defined had to be moved to net461 which doesn't have that type defined
by default.

This change removes the following APIs:
- TypeSymbol.BaseType
- TypeSymbol.Interfaces
- TypeSymbol.AllInterfaces
- NamedTypeSymbol.TypeArguments
- TypeParameterSymbol.ConstraintTypes

These APIs simply delegate the work to APIs with "NoUseSiteDiagnostics" suffix and are not supposed to be used inside compilers. They were left in the code-base because they used to be part of the public surface and the names were nicer, but they are no longer part of the public surface.

* Rename Foo to Goo

* Rename ifoo to igoo

* Add back BOMs

LocalFunctionTypeParameterSymbol isn't necessary as long as the functionality around type constraints can be moved into a shared base class between ordinary method symbols and local function symbols.
In fact, it would be very useful to have a shared base symbol aside from MethodSymbol between local functions, lambdas, and other source method symbols. To this end, I've renamed SourceMemberMethodSymbol to SourceOrdinaryMethodSymbol (since it is just used for ordinary member-level methods), SourceMethodSymbol to SourceMemberMethodSymbol (since it is only used as a base class for member-level method-like functions like ordinary methods, constructors, property accessors, et al.), and added a new abstract class SourceMethodSymbol that is the root of both LocalFunctionSymbol and SourceMemberMethodSymbol. This lets us unify all source method-like symbols under a single base class, SourceMethodSymbol.
The first three commits perform the refactoring, while the last commit actually removes LocalFunctionTypeParameterSymbol and moves the necessary functionality into SourceMethodSymbol. I expect more functionality to be moved over in subsequent PRs.

Fixes #17244

Fixes:#14600
Fixes:#11013
Fixes:#14116
Fixes:#16168

Create an ExecutableCodeBinder when speculatively binding an expression or attribute.
Fixes #14384

We had a few different patterns in our codebase. This unifies all of
them to a single helper. We had not one but two (!) different pieces
of code also trying to create dumps if the assertion failed. I've chosen
to toss both of those, since I believe one of two things to be true:

1. The underlying bugs have been fixed, and it's probable the dumping
   code no longer works.
2. The underlying bugs havent been fixed, but we're clearly not hitting
   that fail code.

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Add ability to associate annotations with type symbols exposed by other symbols. For example, with property's/method's return type, parameter's type, etc. As part of this change, custom modifiers are now handled as an annotation.