Investigate cleanup of CachingSemanticModelProvider

[ToddGrun]

Came up in code review of #77192

Things to consider:

1. Should CachingSemanticModelProvider be merged with it's base class as it's the only class in the product derived from SemanticModelProvider?
2. Should CachingSemanticModelProvider be made static as it only uses static data?
3. Should the Clear* methods be reconsidered? As part of that PR, it was stated that the CompilationEnd notification is intentionally no longer being sent. This is the notification that triggers the Clear(Compilation) method, so we should either reconsider sending the CompilationEnd notification or remove the Clear(Compilation) method from CachingSemanticModelProvider as it's misleading in it's current form.