Make async state machine cleanup hoisted locals before calling SetResult to prevent excess GC roots

… for active file analysis scope.

The compiler merged support for top-level symbol nullability in
dotnet#39498, so we can now delete our
own wrappers that were performing the same thing. This is a mostly
mechanical change.

For places where we were calling LookupSymbols or ClassifyConversion,
those places previously called .WithoutNullability() since the compiler
API would have thrown if we gave it our wrappers, but the APIs had
otherwise no way to pass top-leve nullability. That was an accepted
oversight at the time, and the belief is passing in the full symbol
will generally be more correct.

In the deepest of ironies, the extension methods we had to wrap
top-level nullability was implicitly hiding a potential null reference.
Removing the extension methods is now flagging this, so this fixes it
up. The handling isn't ideal: if we don't have a Task<T> we won't
necessarily wrap, but that's not something really worth going out of
our way to support -- most code doesn't.

This clarifies why we are stripping the nullability, and replaces it
with the "not null" instead of "oblivious" which generates to the same
code but is a bit more correct if something else tried to operate on
the type symbol.

…ctly

We weren't ensuring the the tuple elements had top-level nullability
if we inferred the pattern was itself using nullable.

Visual Studio Code Tasks

* Refactoring

* Refactoring

* Refactoring

* Feedback

* Misc feedback

* Use ValueTask instead of custom task caching

…wrappers

Remove the nullable wrappers from the Workspaces layer

Enable nullability checking in more of our collection types

…203.17 (dotnet#40133)

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.19603.17

Fixes dotnet#39975

* Make finding checksum async
* Clean up TryRunCodeAnalysisRemoteAsync

Nullable collections

After CI image was updated with 16.4 Preview 5 some REPL tests started
failing consistently. dotnet#40160

Skip failing C# REPL integration tests.