[libcxx] makes `iterator_traits` C++20-aware

* adds `iterator_traits` specialisation that supports all expected
  member aliases except for `pointer`
* adds `iterator_traits` specialisations for iterators that meet the
  legacy iterator requirements but might lack multiple member aliases
* makes pointer `iterator_traits` specialisation require objects

Depends on D99854.

Differential Revision: https://reviews.llvm.org/D99855