[libc++] Remove a bunch of conditionals on _LIBCPP_DEBUG_LEVEL

Instead of providing two different constructors for iterators that
support the debug mode, provide a single constructor but leave the
container parameter unused when the debug mode is not enabled.

This allows simplifying all the call sites to unconditionally pass
the container, which removes a bunch of duplication in the container's
implementation.

Note that this patch does add some complexity to std::span, however
that is only because std::span has the ability to use raw pointers
as iterators instead of __wrap_iter. In retrospect, I believe it was
a mistake to provide that capability, and so it will be removed in a
future patch, along with the complexity added by this patch.

Differential Revision: https://reviews.llvm.org/D126993