Support for Clang with MSVC2017

Version 6 of Clang-cl is now compatible with MSVC2017 and its libraries. For detecting this toolchain, QtCreator requires an additional check for Abi::WindowsMsvc2017Flavor in detectClangClToolChain.

After adding this check (diff attached) to the current master QtCreator autodetected LLVM + MSVC2017 and successfully builds Qt/C++ applications (at least with QMake, I guess QBS requires some more work).