Use VS2015 Project Support for Natvis to eliminate the need to manually install clang native visualizer

This is the clang equivalent to llvm commit 264601. When using Visual Studio 2015, cmake now puts the native visualizers in llvm.sln, so the developer automatically sees custom visualizations.
Much thanks to ariccio who provided extensive help on this change. (manual installation still needed on VS2013).

llvm-svn: 264603