When we ship LLVM 16, <ranges> won't be considered experimental anymore.
We might as well do this sooner rather than later.
Differential Revision: https://reviews.llvm.org/D132151
Also fix `ranges::stable_sort` and `ranges::inplace_merge` to support
proxy iterators now that their internal implementations can correctly
dispatch `rotate`.
Differential Revision: https://reviews.llvm.org/D130758