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.

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.

Ensure complete information is consistently captured in a bound tree for an explicit C# conversion implicitly applied by compiler. (#22703)

Fixes #7299. Fixes #19878

* Add tests for basic `private protected` functionality.
* Constrain private protected to language version 7.2.
* Add tests for VB, and adjust existing VB tests as needed.
* Add test for SyntaxFacts production of source for private protected.
* Add VB tests corresponding to all C# tests for private protected
* Mention `private protected` in the compiler test plan.

* Rename Foo to Goo

* Rename ifoo to igoo

* Add back BOMs

Fixes #18985

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

Merging on behalf of @alrz
Thanks for the contribution!

Merging on behalf of @reaction1989
Thanks for the contribution

* simplify partial method return type syntax finder tests

* Bug fixes and API use changes

* address first review

* better comment and compiler trait on class

* fix tests, pull entrypoint check out into local function

* unify diagnostics for intvoid or task mains

* add more tests, change error text

* address review

* finish tests

* be explicit on language version in tests

* add another test, fix typo, add diagnostics even when they aren't likely to exist

* rewrite error->warning code, add more cases to tests

* change test language versions

* formatting on comments and adding WithLocation to tests

* fix formatting for some tests

* fix some formatting and added withlocation

* fix withlocation

* attempt to get main entrypoints working

* async main codegen

* remove garbage file

* fix vb tests

* add some comments

* moduleBeingBuilt can sometimes be null apparently

* fix style nits

* more whitespace fixes

* address feedback

* automated style fixer

* move most work out into the constructor

* protect against null in error condition

* add comment

* address cston review

* autoformat

* use cast

* check module being built

* add another object cast

* address review

* check for old cases where async methods called Main also exist

* more suggestions

* autoformat

* mostly cleanup

* use shorter Count and Single

* remove superfluous diagnostics add

* check exit code / revert to old entrypoint finding code

* remove language version check in entrypoint compiler

* fix debug.assert

* switch to assert equal

* codereview

* add more tests; change conditions for test execution

* fix up emit methods

* narrow down error location

* ammend GetAwaitableExpressionInfo to return a bound call to GetAwaiter().GetResult()

* more comments

* find entrypoint methods uses the async binder

* autoformat

* use diagnostics from async binder

* relocate tests

* fix suggestions for review 8346

* add back no-async tests with better location

* only allow named types

* remove empty file; remove null checks from helper

* forgot to add csproj file

* emit into warning diagnostics instead of regular diagnostics

* autoformat

* remove unused file from merge

* use diagnostic locations

* move some methods around, split some tests up

* fix non-returning async tests, remove bool return from check availability

* add params test

* style and expected output fix

* switch back to null because default emits warnings

* add partial method tests

* fix locations for unix

* better partial syntax tests

* use expected length of 0

* use default length of 0 for expected output

* check implementation symbol

* Writing attribute to metadata

* Reading attribute in PE symbols

* Added more tests for lambdas, delegates, and other types

* Use ReadOnlyAttribute instead

* Fix build break

* Fix Failed Tests

* Enable back disabled tests

* Fix more tests

* Ban usage of ReadOnlyAttribute in source

* Rename System.Runtime.InteropServices.ReadOnlyAttribute to System.Runtime.CompilerServices.ReadOnlyAttribute

* Clean up

* Address CR comments

* More PR feedback

* Responded to PR comments on #17894

* Added more tests, and supporting 1.0, 2.0, 3.0, etc... versions

* Clean up

NOTE: binding of `ref readonly` parameters is NYI