clang: Restore support for clang-cl on non-Windows hosts

The frontend variant detection logic added by commit 53fbe23f3f (clang:
introduce CMAKE_<lang>_COMPILER_FRONTEND_VARIANT, 2019-02-20,
v3.15.0-rc1~41^2~8) assumes that `clang-cl` only runs on a Windows host.
It is also available on non-Windows hosts.  Fix the condition.

Fixes: #19544
1 file changed