[libc++] Use addressof in forward_list.

This addresses the usage of `operator&` in `<forward_list>`.

(Note there are still more headers with the same issue.)

Reviewed By: #libc, Quuxplusone, ldionne

Differential Revision: https://reviews.llvm.org/D112660

GitOrigin-RevId: f7345de64fd2d6c5528e940a8aed1a9b95b46183
5 files changed