[clang] [HIP] Define `_OPENMP` on the device for mixed OpenMP CPU compilations (PR #176791)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> What was the compilation command line by CMake? I guess users pass hipcc as C/CXX compiler which is wrong in the first place.

Yes, as a hip compilation you get `clang++ -x hip -fopenmp CheckVersino.cpp`  which is all that's required

https://github.com/llvm/llvm-project/pull/176791