CodeFixes that alter AdditionalFiles do not save the changes

[dpoeschl]

Version Used: VS 15 Preview 4

Steps to Reproduce:

1. Enlist in Roslyn, under source control
2. Change something from internal to public
3. Use the Public API analyzer/fixer to add the newly public thing to the PublicAPI.Unshipped.txt file

Expected Behavior: PublicAPI.Unshipped.txt should be saved, and show as changed under source control

Actual Behavior: PublicAPI.Unshipped.txt is not saved until you Save All in VS.

[dpoeschl]

Some potential psychic debugging from @Pilchie elsewhere: "Sounds like we're missing a dispose on an invisible editor somewhere"

[dpoeschl]

Also /cc: @tmeschter

[jasonmalinowski]

Oh, hey, turns out it's dotnet/project-system#2160 that I filed after investigating. 😄