Add workaround for AsyncSemaphore disposal problems

Conflicts:
	docs/compilers/CSharp/Compiler Breaking Changes - post VS2017.md
	src/Compilers/CSharp/Portable/Errors/ErrorCode.cs
	src/Compilers/VisualBasic/Portable/VBResources.resx

disabling ShowGenericTypes3

The test is flakey, fails in microbuild

Use MicrosoftSHA2 cert for signing

Add dev15.3-preview1 branch badge

This is a workaround for #18563. See #19042 for a follow-up task to remove
this workaround.

Improve the warning span given when no 'await's are found in an async lambda.

Bump NuGet version for 15.3 preview 2

Enc tests (#18893)

* remove unnecessary tests

* pattern matching tests

Include generic interfaces in implements clause completion

Show diagnostic nodes in Solution Explorer for CPS-based projects

Guard access to shared resource WordSimilarityChecker.s_pool

Add a language service to find entry points that can be accessed from the CPS project system.

Fix logic that was making us unify different local functions if their signatures matched.

Changes to pattern matching to provide an API more suitable for the platform.

Disabling tests due to flakiness

Classify 'var' in patterns as a keyword.

Support ExperimentalAttribute

PDB fixes and testing

Always show snippets should not exclude non-snippet items