clang on windows (not clang-cl) does not accept /await as argument

I just tried to add cppcoro as a vcpkg to my project that is build using raw clang.exe.

However the cppcoro CMake script doesn't properly check for the normal clang and thinks it is clang-cl, and then adds the "/await" command line argument. Ofcourse clang treats that as a file name.

I only need std::generator, that is really easy to make myself, so this ticket is just a notification.
