[libc++] `ranges::ends_with` should forward to `equal`

[philnik777]

equal forwards to memcmp for trivially equality comparable types, which is an order of magnitude faster than the generic loop. We should make use of that optimization in ends_with.

[xiaoyang-sde]

ranges::ends_with invokes ranges::start_with if the iterators model std::bidirectional_iterator, and invokes ranges::equal otherwise.

It seems that the implementation of ranges::start_with relies on ranges::mismatch, which, in turn, is implemented using a generic element-wise comparison. I believe it's possible to optimize ranges::starts_with using ranges::equal.

#84570

[changkhothuychung]

I would like to try this if this is available.

[ldionne]

I don't think that optimizing std::starts_with is sufficient to optimize this though, since ends_with calls starts_with with reverse_iterator. We'd have to unwrap the reverse_iterator if it's wrapping a contiguous_iterator first, I think.