Enable support for doing official builds via build.ps1.

Update CreateProjectInfo to accept relative analyzer paths

Use a linear algorithm to find the best candidate

Merging on behalf of @alrz
Thanks for the contribution!

Pass the correct configuration arg

Revert "Merge pull request #18383 from CyrusNajmabadi/fastInitialTagg…ing"

# Conflicts:
#	src/Compilers/CSharp/Portable/Binder/Semantics/Operators/BinaryOperatorOverloadResolution.cs
#	src/Compilers/CSharp/Portable/Binder/Semantics/Operators/UnaryOperatorOverloadResolution.cs

Fix signed build break

This reverts commit 054509fd, reversing
changes made to fc11e8ad.

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.