[libc++] [test] Don't assume iterators are class types.
In particular, span<int>::iterator may be a raw pointer type
and thus have no nested typedef iterator::value_type. However,
we already know that the value_type we expect for span<int> is just int.
Fix up all other iterator_concept_conformance tests in the same way.
Differential Revision: https://reviews.llvm.org/D101420