[libc++] Fix unintended ABI break in associative containers with reference comparators (#118685)

While reference comparators are a terrible idea and it's not entirely
clear whether they are supported, fixing the unintended ABI break is
straightforward so we should do it as a first step.

Fixes #118559

GitOrigin-RevId: 2393ab65edda14cc416068ebee0b512228723877