This both removes our flaky dependency on a file share and sets us up to
get updates for the package.

Fixes #18556 - Invalid code when implementing interface through explicitly-implemented property

Fixes #16195
This is a language change for 7.1. See https://github.com/dotnet/csharplang/issues/154.

Fix partial method instrumentation

Defer creating a compilation if it is not needed.

Improve performance of tree serialization.

Order 'Remove Unused Variable' after 'Add Using'

Remove unnecessary delegate allocations when calling back to analyzers

Other call sites appear to have been adapted already.
Fixes https://devdiv.visualstudio.com/DefaultCollection/DevDiv/_workitems?_a=edit&id=266237

Crash the compiler if we are trying to emit a method definition token but end up with a handle that does not correspond to a method definition.

* Improve path completion providers

* Make GAC completion faster and cancellable

* Move FileSystemCompletionHelper.cs

* Move LoadDirectiveCompletionProviderTests.cs

* PR feedback and fix completion for 'X:' paths

Fix crash in AddUsing when changing casing of a typename.

Fix a small memory leak related to batching project loads

Fix configurations so that project will load in 15.3

Use a wait-free fast path in AsyncLazy<T>.GetValueAsync

Do a better job preserving blank lines when we convert code to using an object initializer.

Do not allow 'add nuget reference' for projects that we fail calling into Nuget for.

This PR updates all mentions of OSX/OS X to macOS in the README.