[WIP] Added cache invalidation policy when a solution changes

[TIHan]

We had a bug where we didn't clear out FCS's caches when a solution was changed. This means, for example, switching configurations between Debug to Release, and vice versa, would cause a new solution to be created with completely different project ids; it would not clear the old projects from FCS out of its cache, effectively keeping them around.

This PR addresses the issue. It also adds a new API, InvalidateProject, to the compiler service that allows you to clear out a specific project from the incrementalbuilder cache. The API is used in FSharp.Editor when a solution has removed projects from itself.

This also does various cleanup; unified handling of single files with normal projects which means one cache in FSharpProjectOptionsManager.

[dsyme]

Just one minor change requested

I'd be interested to know if this can be unit tested?

[TIHan]

Good question on the testing part. I think it's possible if I only split out the functions. I'll see if it can be done.

[cartermp]

Minor suggestions and a question

[TIHan]

Marking this as WIP as we first need to fix the reference counting problem with IB (IncrementalBuilder) by pushing disposing of type providers and ilbinary readers down the TcImports' Finalizer.

[TIHan]

I still would like to finish the solution cache invalidation strategy; but it's a little tricky. My fear is that it may make behavior worse. It seems everytime I want to remove something from the incrementalBuildersCache, it can make things worse because it causes cache thrashing; basically it can get into a state where it is constantly removing from the cache and rebuilding when it doesn't need to, multiple times. I think it has to do with IProjectReference and background builds; it's still a bit unclear to me what I can do without hurting existing behavior. I don't want to hurt current users of FCS.

[cartermp]

@TIHan I'm going to close this since it appears #9124 accomplishes the same goal.