Support for MSVC v141 toolset

The MSVC v140 and v141 compiler toolsets are compatible, but it’d still be good to have full support for the former. Right now we have to enable the v140 toolset in VS2017 to make LLVM/Clang integrate with it, it seems. But what if we want to use the newer toolset with LLVM? Please try to work with Microsoft to fix this. Thank you.