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

[ldionne]

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

[frederick-vs-ja]

I think if one use #pragma pack to create misaligned elements, such change will make span over them stop working. Although I'm not sure whether such use is supported now.