Clang: Do not mistake clang-cl 6.0 for GNU-like clang

The check added by commit v3.10.0-rc2~2^2 (Clang: Diagnose unsupported
GNU-like clang targeting MSVC ABI, 2017-10-10) is incorrectly detecting
clang-cl 6.0 as GNU-like.  Currently cmake is testing if the clang
compiler accepts `--version` to see if it accepts GNU style flags.
However, with the latest llvm snapshot this also works for clang-cl:

    > clang-cl --version
    clang version 6.0.0 (trunk)
    Target: x86_64-pc-windows-msvc
    Thread model: posix
    InstalledDir: C:\Program Files\LLVM\bin

So instead we should use the `/?` flag which fails with clang but
works with clang-cl:

    > clang-cl /? &> /dev/null; echo $?
    0
    > clang /? &> /dev/null; echo $?
    1

Fixes: #17518
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