[libc++] Make __wrap_iter constexpr

`__wrap_iter` is currently only constexpr if it's not a debug built, but it isn't used in a constexpr context currently. Making it always constexpr and disabling the debugging utilities at constant evaluation is more usful since it has to be always constexpr to be used in a constexpr context.

Reviewed By: ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D114733