[libc++] Unwrap reverse_iterator<reverse_iterator<Iter>> in __unwrap_iter

Simplify the implementation of `std::copy` and `std::move` by using `__unwrap_iter` and `__rewrap_iter` to unwrap and rewrap `reverse_iterator<reverse_iterator<Iter>>` instead of specializing `__copy_impl` and `__move_impl`.

Reviewed By: ldionne, #libc

Spies: wenlei, libcxx-commits

Differential Revision: https://reviews.llvm.org/D127049