[libc++] [LIBCXX-DEBUG-FIXME] <span>, like <string_view>, has no use for debug iterators.

[libc++] [LIBCXX-DEBUG-FIXME] <span>, like <string_view>, has no use for debug iterators.

A span has no idea what container (if any) "owns" its iterators, nor
under what circumstances they might become invalidated.

However, continue to use `__wrap_iter<T*>` instead of raw `T*` outside
of debug mode, because we've been shipping `std::span` since Clang 7
and ldionne doesn't want to break ABI. (Namely, the mangling of functions
taking `span::iterator` as a parameter.) Permit using raw `T*` there,
but only under an ABI macro: `_LIBCPP_ABI_SPAN_POINTER_ITERATORS`.

Differential Revision: https://reviews.llvm.org/D101003