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

[Ye Luo via cfe-commits]

ye-luo wrote:

Could you please provide a minimal reproducer to explain the issue. Based on the limited description, it sounds like user tries to compile codes with hip source code with OpenMP threading and the hip part was reactive to `__OPENMP__`. It is complicated to ask hip mode supporting full OpenMP logics (not just predefined macro but also API header files, directives). My recommendation will be asking users to spin off the hip code as separate functions in cpp file without openmp, the old cpp file then doesn't need to be compiled in hip mode.

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