[MSBuildWorkspace] ProjectReferences vs. AllProjectReferences return the same count despite projects not existing in the solution.

Version Used: master, 16.6.0, etc

Steps to Reproduce:

1. Create a project A and a project B
2. Have A reference B
3. Create a solution X and add only A to it
4. Use MSBuildWorkspace and OpenSolutionAsync()
5. Compare A's ProjectReferences to AllProjectReferences

Expected Behavior:

They should be different, because solution X only has A in it.

Actual Behavior:

They are the same because the way a solution is loaded causes project references to be loaded into it even if they're not actually in the solution. See here:

https://github.com/dotnet/roslyn/blob/2a69ac38995a07f4333158b76d2f6e780868401a/src/Workspaces/Core/MSBuild/MSBuild/MSBuildProjectLoader.Worker.cs#L138-L172