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

[via cfe-commits]

EricWF wrote:

@philnik777 Last we spoke, the design work around the customization points was still ongoing. Have we locked it in now?

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