MSBuildWorkspace doesn't handle multi-targeted unloadable projects correctly when resolving references

It'd likely be simpler to allow `LoadProjectFileInfosAsync` and `LoadProjectFileAsync` to load any project, not just C# and VB. For unloadable projects it would only fill output file paths `ProjectFileInfo` and set the `Language` to null and the caller would handle it appropriately.

Tracks TODO in code

Start by locating the TODO and the LoadProjectFileInfosAsync and LoadProjectFileAsync entry points in the Roslyn repository. Trace how multi-targeted unloadable projects are handled during reference resolution; done means those projects provide output file paths with a null language and the caller handles them appropriately.

Written by the indexing model from the issue text.