[pstl] Initial implementation of OpenMP backend, on behalf of Christopher Nelson nadiasvertex@gmail.com

Phabricator Review:
https://reviews.llvm.org/D99836

A couple of parallel patterns still remains serial - "Parallel partial sort", and "Parallel transform scan" - there are //TODOs in the code.

GitOrigin-RevId: 6069a6a5049497a32a50a49661c2f4169078bdba
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