[libc++] Assume that __wrap_iter always wraps a fancy pointer.

Not only do we conscientiously avoid using `__wrap_iter` for non-contiguous
iterators (in vector, string, span...) but also we make the assumption
(in regex) that `__wrap_iter<_Iter>` is contiguous for all `_Iter`.

So `__wrap_iter<reverse_iterator<int*>>` should be considered IFNDR,
and every `__wrap_iter` should correctly advertise contiguity in C++20.

Drive-by simplify some type traits.

Reviewed as part of https://reviews.llvm.org/D102781