[libcxx] [libcxxabi] [llvm] Adding OpenMP Offloading Backend for C++ Parallel Algorithms (Rebased #66968) (PR #122180)

[Johannes Doerfert via llvm-commits]

jdoerfert wrote:

Locally, ninja check-cxx works fine, some of the pre-commit configurations do not set fopenmp but end up including the backend openmp.h. I am not sure why exactly. @ldionne  Thoughts?

https://github.com/llvm/llvm-project/pull/122180