Sadly, I realized that CMAKE_<LANG>_SIMULATE_ID is not really helpful.
Whether I use clang or clang-cl it always evaluates to MSVC.
CMAKE_<LANG>_COMPILER_FRONTEND_VARIANT on the other hand only evaluates to MSVC if using clang-cl and to GNU for clang.
So what I really need to pass options to Clang with the correct format is CMAKE_<LANG>_COMPILER_FRONTEND_VARIANT.
Is there any reason, why that variable is supposed to be a CMake-internal one?
I would suggest to officially expose it to users and to provide proper documentation to both variables, because the documentation for CMAKE_<LANG>_SIMULATE_ID seems not to reflect what that variable really shows. (It seems more as if CMAKE_<LANG>_SIMULATE_ID determines if I am calling Clang from a Visual Studio prompt but not in what driver-mode it is running.)