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

[Anton Rydahl via cfe-commits]

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AntonRydahl wrote:

I have written a section about this in the `libc++` documentation. Do you think the description is sufficiently detailed?

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