[libc++][pstl] Generic implementation of parallel std::is_sorted (#176129)
This PR implements a generic backend-agnostic parallel `std::is_sorted`
based on `std::transform_reduce`.
While this approach is suboptimal comparing a direct backend-specific
implementation, since it doesn't support early termination and requires
a reduction operation, it does show speedup when the dataset is large
enough and the comparator is not absolutely trivial.
Parent issue: #99938