Improved Visual Studio visualization of OpaquePtr

Create a special visualizer for OpaquePtr<QualType> because the 
standard visualizer doesn't work with OpaquePtr<QualType>
due to QualType being heavily dependent on traits to be pointer-like.

Also, created an identical visualizer for UnionOpaquePtr


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@272531 91177308-0d34-0410-b5e6-96231b3b80d8