[libc++] [LIBCXX-DEBUG-FIXME] Our `__debug_less` breaks some complexity guarantees.

`__debug_less` ends up running the comparator up-to-twice per comparison,
because whenever `(x < y)` it goes on to verify that `!(y < x)`.
This breaks the strict "Complexity" guarantees of algorithms like
`inplace_merge`, which we test in the test suite. So, just skip the
complexity assertions in debug mode.

Differential Revision: https://reviews.llvm.org/D101677