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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> So the error is caused by your `c++` not supporting `__has_feature` presumably. However I have no clue why that would happen due to this patch. I believe in a few days @jdoerfert will flip the switch and we won't even allow `libomptarget` to be built without using the runtimes support so this might not be worth thinking about too hard.
> 
> Any suggestions for reproducing it?

Oh, I think I get it. We're adding the resource directory include to the include paths. This is not conflicting with the ones from GCC. Honestly we only need this for `ompt` stuff which I could just do directly I suppose.

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