clang-cl msvc mode on non-win32 host is broken in cmake3.15
I use clang-cl to build my projects on linux and macOS. My toolchain file is https://github.com/wang-bin/cmake-tools/blob/master/windows.clang.cmake It works fine for cmake3.10~3.14. But cmake 3.15 assumes CMAKE_C_COMPILER_FRONTEND_VARIANT is GNU if host os is not win32, it's not true.