[libc++] Remove usage of `_LIBCPP_DEBUG` in `__comp_ref_type` and replace with...

[libc++] Remove usage of `_LIBCPP_DEBUG` in `__comp_ref_type` and replace with `_LIBCPP_DEBUG_LEVEL`

In libc++, checking specific `_LIBCPP_DEBUG_LEVEL` levels is used everywhere except in `comp_ref_type.h`. `_LIBCPP_DEBUG` is meant as a user-facing option, and internally libc++ should be checking the value of `_LIBCPP_DEBUG_LEVEL`.

The definition of `std::__debug_less` doesn't need to be hidden behind the macro, we can unconditionally expose it. It will be unused by `__comp_ref_type` unless debug mode is enabled.

This was suggested in D118940.

Reviewed By: #libc, philnik, Quuxplusone, ldionne

Differential Revision: https://reviews.llvm.org/D118950