Fix file-name to ID-mapping code in solution.

[CyrusNajmabadi]

The only "bug" I see is what you called out in the add/remove documents where they're creating a new file name -> ID map from the original state, rather than the state it's built up so far while looping for each set of documents per project. That indeed looks like a bug, so do file a bug for that (or poke me to do it.)

#71257 (review)

Also

I agree you have maintained the behavior, but I strongly suspect that original behavior is a bug, and in VS we never notice since we'll only ever see all the documents applying to a single project. I'm quite surprised we didn't have a test for that, so if you don't want to investigate as a part of this PR please do file a tracking bug for this.

#71257 (comment)