Move the implementation of RQNames down to hte Feature-layer.

This wasn't previously flagged because this only comes into play if there is an error.  As
written the code will display "$echo isn't a variable" instaed of the actual error.

Fix issue with definite-assignment checking in 'inline-decl' with 'var-typed' locals.

Merge dev15.2.x to master

Removed some unnecessary codegen from async state machine.

Add test for issue https://github.com/dotnet/roslyn/issues/18844

Remove unnecessary parsing logic.

Disabling flakey CodeActions.MoveType tests

Fixes #18257

In order to leverage LUT testing we need to have both of the
follownig packages in our build.

- xunit.runner.desktop
- xunit

Both of these include xunit.abstractions.dll in different forms: content
and reference respectively.  This creates a double write violation in our
build.

I've spoken with the xunit team and at this point they believe the
feature is executing as designed.  The content use in xunit.runner.desktop
is meant to support 1.X clients who can't reference the package.

I'm still working with them to find a better way to exclude this but
for now this will remove the race in our build.

Issue #18573 will remain open to track the handling here and find
a better solution.

There is no good method today for executing windows commands with
dynamically built argument lists in Powershell.  This change attempts
to address that by introducing Exec-Command which uses the .NET
Process API directly.  This makes it straight forward to build windows
commands that rely on windows style parsing and output

Pool more data while pattern matching.

Merge dev15.1.x to dev15.2.x

It seems like the reference set for CoreCLR tests is sometimes different, resulting
in CI failures. This is likely due to the lazy initialization of static MetadataReferences
in TestBase without thread synchronization. This change adds Interlocked calls where
necessary and adds a comment warning about when more Interlocked calls may be
needed.

This is a test-only change.

Fix build due to bad merge with CreateCompilationWithMscorlib

Fixes #17971