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

[Shilei Tian via cfe-commits]

shiltian wrote:

> I won't consider a hip compiler or clang in hip mode as a valid OpenMP compiler hence doesn't need to follow OpenMP spec.

+1. That's why I asked why the user sets a "HIP compiler" (most likely `hipcc`) as a regular CMake C++ compiler and asked it to go through the CMake compiler OpenMP tests.

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