[libc++] Use a __bounded_iter flavor in std::array

[ldionne]

Just like we do for std::string_view and std::span. But we can hardcode the size information in the iterator type, leading to 2 pointers instead of needing 3 pointers like __bounded_iter.

[philnik777]

We could also make it a single pointer and a counter. That would allow us to have a smaller fancy pointer or even bit-pack the count into the lower bits for small N.

[ldionne]

For information this will be blocked on #74482.