[libc++] Fix algorithms which use reverse_iterator

This adds a C++20-version of `reverse_iterator` which doesn't SFINAE away the operators for use inside the classic STL algorithms. Pre-C++20 `_AlgRevIter` is just an alias for `reverse_iterator`.

Reviewed By: var-const, #libc

Spies: huixie90, libcxx-commits

Differential Revision: https://reviews.llvm.org/D128864