[libc++] __iterator/readable_traits.h isn't standalone

`__iterator/readable_traits.h` can't be used by itself, intantiating `iter_value_t` requires `__iterator/iterator_traits.h`. `readable_traits.h` can't include `iterator_traits.h` though because `iterator_traits.h` requires `readable_traits.h`.

Move `iter_value_t` to `__iterator/iterator_traits.h` so that both headers can work standalone.

Reviewed By: Mordante, #libc

Differential Revision: https://reviews.llvm.org/D153828