[clang] [openmp] [Libomp] Place generated OpenMP headers into build resource directory (PR #88007)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> I should have mentioned, I'm building with GCC 9.3.0. Most of my team uses this compiler.

Yeah, that's fine for building LLVM but in the future we'll probably require building the offloading runtime with an up-to-date clang. I think for now I can just restrict this patch to runtimes builds.

https://github.com/llvm/llvm-project/pull/88007