[pstl][openmp] Patch is ready for PSTL openmp backend

I have uploaded a patch for the PSTL openmp backend. This patch is complete, as far as I know. Please look at https://reviews.llvm.org/D99836 and let me know what is needed.

I note that clang-tidy fails, but the failures appear to be spurious. I’m not sure how to work around those problems.

-={C}=-

FWIW: Clang-tidy doesn’t run on libcxx/; I believe you don’t want to run it on pstl/ either. But I don’t know what magic makes that happen. It’s probably in some build-system-configuration file somewhere in the repo; I just don’t know where.

–Arthur

I have tried to look around and find it, but it’s not obvious. Who might I ask?