[libc++] Consider using `blqs::sort` algorithm for trivially relocatable types for `std::sort`

https://quuxplusone.github.io/blog/2026/06/05/branchless-sorting/
https://tiki.li/blog/blqsort

Start by reading the two linked articles and then locate libc++'s std::sort implementation and its handling of trivially relocatable types. Determine whether blqs::sort is applicable and what libc++ performance tests or benchmarks cover this path. Done means a reviewed decision and, if appropriate, an implementation backed by measurements and tests.

Written by the indexing model from the issue text.