[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