Improved Visual Studio 2015 visualization of SmallVectorImpl

When visualizing small vectors in VS2015, show the first few elements in the DisplayString instead of the size. For example, a SmallVector of DeclAccessPair will visualize like

  {public typename ...Ts, public typename U}

The visualization in VS2013 remains the same because we continue to include the old visualizer with a lower-than-default priority of MediumLow, and the same SmallVector would continue to be visualized as

  {size = 2}

llvm-svn: 272525