[libc++] Fix a hard error in `contiguous_iterator<NoOperatorArrowIter>`.

Evaluating `contiguous_iterator` on an iterator that satisfies all the
constraints except the `to_address` constraint and doesn't have
`operator->` defined results in a hard error. This is because
instantiating `to_address` ends up instantiating templates
dependent on the given type which might lead to a hard error even
in a SFINAE context.

Differential Revision: https://reviews.llvm.org/D130835