[clang] [llvm] [OpenMP][clang] Set num_threads 'strict' to unsupported on GPUs (PR #160659)

[Robert Imschweiler via llvm-commits]

ro-i wrote:

I think `OpenMPClauses` might be the most appropriate warning group? `OpenMPTarget` would be the other choice, ig, but it seems to be rather used in other situations

https://github.com/llvm/llvm-project/pull/160659