[clang-tools-extra] Adding Separate OpenMP Offloading Backend to `libcxx/include/__algorithm/pstl_backends` (PR #66968)

[Louis Dionne via cfe-commits]

https://github.com/ldionne requested changes to this pull request.

This is starting to look really good! Note to self: still need to dive into the implementation of most algorithms, but we discussed `for_each` and offloading conditions in some details.

https://github.com/llvm/llvm-project/pull/66968