Add support for merging preview items across multiple operations.

Provide a way for code actions to supress diagnostics in their previews.

Remove unnecessary casts.

This is useful for code actions that will make changes that can't be
represented in the Roslyn snapshot model.  Because of this, the
preview may end up showing errors that teh code fix doesn't want to
be shown (because after the code fix actually applies, no error will
occur).

Silence "No source files" build warning

Also, wrap all code action operations with an undo transaction.
This way we will attempt to undo all changes as one user undo
action.

CSharpAnalyzers.Vsix.vbproj was pulling CSharp targets and csc.exe, so their supressed warnings were not being respected.
Suppressed this warning in CSharpAnalyzers.Vsix.csproj.

 Asynchronously produce the SpellChecker used for fuzzy matching.

Remove EventListener crash workaround

We've turned off AppDomain isolation in https://github.com/dotnet/roslyn/pull/8672 and https://github.com/dotnet/roslyn/pull/8534.

Merge stabilization into master

Use common output directory for Samples.

[Ask Mode] Fix IOperation crash caused by passing null for ConstantValue when creating Lite…

Search projects in parallel.  Cancel any parallel work once enough results are found.

Ask Mode request: Prevent DeriveArgument from throwing ArrayIndexOutOfRangeException when presented with semantically malformed bound trees.

Prevent DeriveArgument methods from trying to access parameter symbols that don't exist.

Disable DocCommentTest Include_Cycle_WRN_XMLDocInvalidXMLFragment

This time it is caused by BontrolVariable bounf to BoundBadExpression.
Also improved the robustness in the Condition property.

EnC: Fix handling ByRef parameters in anonymous delegates

Fix reset interactive build cancellation

Do not offer to create p2p references if the project system will not allow those references.

If we see a spike in this data we can use that to help realize that
we probably have some sort of bug/mismatch in our code to serialize/deserialize.

Prevent DeriveArgument from throwing ArrayIndexOutOfRangeException when presented with semantically malformed bound trees.

Use common output directory for Samples.
Fixes #8638

Fix an issue in the test caching layer

Update doc comment tests that contain XML comments with paths