[libcxx] contiguous iterator concept: don't require pointer or complete element types

`contiguous_iterator` requires the iterator type passed is either a
pointer type or that the element type of the iterator is a complete
object type. These constraints are not part of the current wording in
defining the `contiguous_iterator` concept - adjust the concept to
reflect this.

Inspired from discussion at https://reviews.llvm.org/D108645.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D108855