[libc++] Use addressof to fix debug tests.

Fixes the tests added in D110852 for the debug iterators.

Similar issues with hijacking `operator&` still exist, they will be
addressed separately.

Reviewed By: #libc, ldionne, Quuxplusone

Differential Revision: https://reviews.llvm.org/D111564

GitOrigin-RevId: 968e27397caabafdad46a79dee5677f64bc8da89
8 files changed