[libc++] Introduce a escape hatch for the changed behavior of map and set search operations (#183190)

In #155245, we implemented an optimization to std::map and std::set
search operations. That optimization took advantage of something that is
guaranteed by the Standard, namely that the comparator provided to the
associative container is a valid strict weak ordering.

Sadly, some code in the wild did not satisfy this requirement, such as
Boost.ICL: boostorg/icl#51

Since this can have extremely tricky runtime consequences, this patch
introduces a temporary escape hatch for the LLVM 22 release that allows
reverting to the previous behavior. It also explicitly calls out the
change in the release notes, adds some regression tests and adds debug
mode support for catching some of these invalid predicates.

Fixes #183189

GitOrigin-RevId: 5de92856bd6c238eb703202a8934f03480e80ee5