[libc++][NFC] Use __constexpr_memmove instead of copy_n in <__string/char_traits.h> (#85920)

`copy_n` has been used to allow constant evaluation of `char_traits`. We
now have `__constexpr_memmove`, which `copy_n` just forwards to. We can
call `__constexpr_memmove` directly, avoiding a bunch of instantiations.
This reduces the time it takes to include `<string>` from 321ms to
285ms.