Clang: Diagnose unsupported GNU-like clang targeting MSVC ABI

The LLVM/Clang installer on Windows provides a `LLVM/bin` directory
containing `clang.exe` and `clang++.exe` command-line tools that have a
GNU-like command-line but target the MSVC ABI (instead of MinGW).  We
do not support this combination, so diagnose and reject it explicitly.
Tell users what to do to use the `clang-cl.exe` tool instead.

Issue: #16439
3 files changed