Fix building on 15.3

Generate better names for expressions used as parameters.

Avoid allocations due to use of Keys/Values instead of GetEnumerator

Offer enum completion inside an enum member's initializer

Eliminate delegate allocations in PooledObject.Create

Fix support for local functions in "Make Method Asynchronous"

The former always has the meaning we want, the latter has it only
when there are no Returns used anywhere in the same file!

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.