[libc++][spaceship][NFC] Marked `operator<=>` for `move_iterator` and...

[libc++][spaceship][NFC] Marked `operator<=>` for `move_iterator` and `counted_iterator` as implemented

Apprarently implemented as part of:

- `move_iterator` - https://reviews.llvm.org/D117656
- `counted_iterator` - https://reviews.llvm.org/D106205

Reviewed By: #libc, philnik

Spies: libcxx-commits, yaxunl

Differential Revision: https://reviews.llvm.org/D145243