Add a VS "14" msbuild toolset

This allows people to try clang inside MSBuild with the VS "14" CTP
releases.

Fixes PR20341.

Patch by Marcel Raad!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213819 91177308-0d34-0410-b5e6-96231b3b80d8