Implement cpp module backend for Clang on Windows

According to this link it is possible use Clang on Windows to create MSVC-ABI-compatible output.

It can be done using either "clang-cl.exe" or "clang++.exe --target=x86_64-windows-msvc", where the first is command-line compatible with cl.exe and the second with gcc. (If I understand correctly you still need MSVC's link.exe for the linking process).

It would be nice to have the possibility to use Clang on Windows with Qbs.