[libc++][ranges] Implement changes to reverse_iterator from One Ranges Proposal.

Changes in [P0896](https://wg21.link/p0896):
- add `disable_sized_sentinel_for`;
- add `iter_move` and `iter_swap`;
-

[libc++][ranges] Implement changes to reverse_iterator from One Ranges Proposal.

Changes in [P0896](https://wg21.link/p0896):
- add `disable_sized_sentinel_for`;
- add `iter_move` and `iter_swap`;
- add a `requires` clause to the `operator->`;
- add `iterator_concept`;
- check that the `Iterator` template parameter is a bidirectional
iterator;
- add constraints to all comparison operators;
- change the definitions of `iterator_category`, `value_type`,
`difference_type` and `reference` (changes to `iterator_category` were
already implemented).

Also add a few forgotten things to the `reverse_iterator` synopsis
(notably the spaceship operator).

Differential Revision: https://reviews.llvm.org/D120180

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