If you have two projects that reference each other with metadata
references, we should avoid converting both of them into project
references as that causes a circularity in the Workspace layer.
The workspace API correctly throws exceptions if you try to do this,
but the Visual Studio project system layer wasn't checking, and so it'd
call into the Workspace APIs and throw exceptions. Unfortunately this
often resulted in us getting into corrupted states where the data
structures got out of sync, which might later result in other exceptions
too.

This fix isn't entirely correct in that we're not going to re-convert
the metadata reference to the project reference if something else breaks
the circularity; that's better than crashing so this is an improvement
either way.

Fixes https://github.com/dotnet/roslyn/issues/39904
Fixes https://github.com/dotnet/roslyn/issues/43633