Fix signed build break

Correct the reference to the deleted BuildAndTest.proj file to use the build.ps1 script.

Merge master to dev15.6

Add non-test IVT for project system test assemblies

Merge master to dev15.6

Support all roslyn languages in Formatter.FormatAsync(Document)
Fixes #20285

Unify our build logic

Source packages cleanup

Optimize fast path for "Use 'var'" code style

Validating params even if they aren't last
Fixes #18875.

This is to prevent misleading errors like "cannot convert from 'typeA'
to 'typeA'" https://github.com/dotnet/roslyn/issues/18875

* Error CS0503 shows the correct modifier target

Update IDE Test Plan with 15.3 features

Fade out unreachable code, and provide code-fix to remove unreachable code.

Properly push events to the OOP side so that we know when things like solution-closing happens.
Fixes #19768

Intern the strings we create for our SyntaxTree index.

Nugetize shared code

Upgared MSBuild / vswhere versions

Correct the error message in our NuGet package

Remove the AnalyzerManager singleton to prevent storing analyzer exec…

Rename Microsoft.CodeAnalysis.Metadata project to Microsoft.CodeAnalysis.Debugging to match the namespace