Previously, we'd use raw pointers when the debug mode was enabled,
which means we wouldn't get out-of-range checking with std::span's
This patch introduces a new class called __bounded_iter which can
be used to wrap iterators and make them carry around bounds-related
information. This allows iterators to assert when they are dereferenced
outside of their bounds.
As a fly-by change, this commit removes the _LIBCPP_ABI_SPAN_POINTER_ITERATORS
knob. Indeed, not using a raw pointer as the iterator type is useful to
avoid users depending on properties of raw pointers in their code.
This is an alternative to D127401.