Test/clang mingw

[memsharded]

Changelog: Omit Docs: Omit

Tested, all with CMake:

• Windows LLVM/Clang with mingw generator
• Windows LLVM/Clang with Ninja and NMake generators
• Windows LLVM/Clang Using different versions of the VS runtime
• Windows Visual Studio integrated ClangCL
• Windows Msys2 LLVM/Clang targeting native Windows, Mingw Clang toolchain

Windows Visual Studio ClangCL with externally installed LLVM/CLang is impossible with CMake, it seems possible with native Visual Studio.