[libcxx][test] array and basic_string_view iterators are not portably pointers

Fixup tests that believe them to be so. Most notably including some heavy refactoring in `std/iterators/iterator.primitives/iterator.traits/cxx20_iterator_traits.compile.pass.cpp`, which now detects pointers and validates that `iterator_concept` is present only for pointers.

Differential Revision: https://reviews.llvm.org/D117368