[libcxx] adds `cpp17-.*iterator` concepts for iterator_traits

The `iterator_traits` patch became too large for a concise review, so
the "bloat" —as it were— was moved into this patch. Also tests most
C++[98,17] iterator types to confirm backwards compatibility is
successful (regex iterators are intentionally not present, but directory
iterators are due to a peculiar error encountered while patching
`iterator_traits`).

Depends on D99461.

Differential Revision: https://reviews.llvm.org/D99854