[libc++] std::sort strict-weak-ordering checks don't trigger

[ldionne]

We should figure out why this doesn't fail earlier with the strict weak ordering checks, but this can be done in a follow-up.

Originally posted by @ldionne in #77405 (comment)

[var-const]

Writing down the results of a (very) quick preliminary investigation, just for future reference:

• __check_strict_weak_ordering_sorted runs after sorting is done, so the actual OOB access happens before we check the comparator. Not sure if this is right -- naively, I would expect the 100-element check to validate the comparator before it is actually used. It might be that the validation function also wants to check the resulting sorted sequence, but in that case it could be split into 2 functions, one that is run before sorting and one that is run after.
• not 100% sure why the __debug_less comparator check doesn't get triggered, but probably that's because it's a best effort heuristic that just happens not to catch this particular case.