[libc++] Should we add alignment assumptions in std::span iterators?

Just like we added alignment assumptions to `std::vector::begin()` to ease vectorization and other optimizations, we could do the same for `std::span`. Is there value in doing that? Is it legal for us to do that?

I think the answer to both of these questions is "yes", but I'm creating this issue so we can investigate properly.

CC @AbhayKanhere @fhahn