Revert "Revert "[libc++] [P0879] constexpr std::nth_element, and rewrite its tests.""

This reverts commit b6ffece3.

The bug is now fixed (it was a stupid cut-and-paste kind of error),
and the regression test added. The new patch is also simpler than the old one!

Differential Revision: https://reviews.llvm.org/D96084