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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> If possible, it will better asking users fixing their cmake rather than complicating hip mode.

The OpenMP standard says that if `-fopenmp` is passes `_OPENMP` is defined. It's an easy fix if you know what you're doing but I'm unsure if we should be violating this, it's not just this test though it's the relevant bit.

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