Clean up code, use DiagnosticInfo in LibraryImportGenerator, Cache well known attribu...

Clean up code, use DiagnosticInfo in LibraryImportGenerator, Cache well known attribute types in StubEnvironment (#87348)

Remove unused usings and simplify access to CodeAnalysis.Location.

Cache the attributes searched for in incremental stub calculations.

Add warning if unsafe blocks are not allowed.

Removes assert in ComMethodInfo that was throwing when editing files in the test project.

Co-authored-by: Jeremy Koritzinsky <jkoritzinsky@gmail.com>