[clang][HIP][OpenMP] Add warning if mixed HIP / OpenMP offloading

Adds a warning, issued by the clang semantic analysis, if HIP and OpenMP target offloading is requested concurrently.
That is, if HIP language mode is active but OpenMP target directives are encountered.
Previously, a user might not have been aware that target directives are ignored in such a case.

Generation of this warning is (lit-)tested via "make check-clang-semaopenmp".
The warning can be ignored via "-Wno-hip-omp-target-directives".

Differential Revision: https://reviews.llvm.org/D145591