[libc++] Introduce __for_each_segment and use it in copy/move

[libc++] Introduce __for_each_segment and use it in copy/move

This simplifies the code inside copy/move and makes it easier to apply the optimization to other algorithms.

Reviewed By: ldionne, Mordante, #libc

Spies: arichardson, libcxx-commits

Differential Revision: https://reviews.llvm.org/D151265