[libcxx] Implement three-way comparison for std::reverse_iterator

This patch implements operator<=> for std::reverse_iterator and
also adds a test that checks that three-way comparison of different
instantiations of std::reverse_iterator works as expected (related to
D113417).

Reviewed By: ldionne, Quuxplusone, #libc

Differential Revision: https://reviews.llvm.org/D113695