Consider making VisualStudioMetadataReferenceManager.ctor thread safe

[ToddGrun]

The main thread affinity of this ctor should be investigated further to determine if it would be possible to remove that dependency. Tying this to the main thread puts a bit of an onus on the creators of this type to understand this requirement, and the main thread in VS is already bogged down quite a bit, particularly during package/solution load.

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[ToddGrun]

Created as part of the review for #77768 where we had to work around this dependency.