[libc++] Fix __wrap_iter copy-assignment in constexpr contexts

Fixes https://github.com/llvm/llvm-project/issues/52902

In debug mode during constant evaluation the iterator was never assigend. There seem to be no other instances of this bug.

Reviewed By: Quuxplusone, Mordante, #libc, ldionne

Spies: ldionne, libcxx-commits

Differential Revision: https://reviews.llvm.org/D116346