[libc++] Fix reverse_iterator::iterator_concept

Fixes https://github.com/llvm/llvm-project/issues/56504

Reviewed By: ldionne, Mordante, huixie90, #libc

Spies: libcxx-commits, hewillk

Differential Revision: https://reviews.llvm.org/D129794

NOKEYCHECK=True
GitOrigin-RevId: 7912b1f8e7c845a97411cbfc176db56861cdf116
3 files changed