[libc++][ranges] Finish LWG issues directly related to the One Ranges Proposal.

[libc++][ranges] Finish LWG issues directly related to the One Ranges Proposal.

- P1252 ("Ranges Design Cleanup") -- deprecate
  `move_iterator::operator->` starting from C++20; add range comparisons
  to the `<functional>` synopsis. This restores
  `move_iterator::operator->` that was incorrectly deleted in D117656;
  it's still defined in the latest draft, see
  http://eel.is/c++draft/depr.move.iter.elem. Note that changes to
  `*_result` types from 6.1 in the paper are no longer relevant now that
  these types are aliases;
- P2106 ("Alternative wording for GB315 and GB316") -- add a few
  `*_result` types to the synopsis in `<algorithm>` (some algorithms are
  not implemented yet and thus some of the proposal still cannot be
  marked as done);

Also mark already done issues as done (or as nothing to do):
- P2091 ("Fixing Issues With Range Access CPOs") was already implemented
  (this patch adds tests for some ill-formed cases);
- LWG 3247 ("`ranges::iter_move` should perform ADL-only lookup of
  `iter_move`") was already implemented;
- LWG 3300 ("Non-array ssize overload is underconstrained") doesn't
  affect the implementation;
- LWG 3335 ("Resolve C++20 NB comments US 273 and GB 274") was already
  implemented;
- LWG 3355 ("The memory algorithms should support move-only input
  iterators introduced by P1207") was already implemented (except for
  testing).

Differential Revision: https://reviews.llvm.org/D126053