Add xml files for msbuild integration

These files are intended to live in the msbuild toolset directory, which
is somewhere like:

C:\Program Files (x86)\MSBuild\Microsoft.Cpp\
  v4.0\Platforms\Win32\PlatformToolsets\llvm

More work is needed to install them as part of the NSIS installer.

Patch by Warren Hunt!

llvm-svn: 189411