[APINotes] Make an assert in a std::sort call tolerate self-comparisons

libc++ debug mode verifies that a comparator passed to std::sort defines
a strict weak order by calling it with the same element.

See also:

- RFC that introduced the feature:
  https://discourse.llvm.org/t/rfc-strict-weak-ordering-checks-in-the-debug-libc/70217

- `strict_weak_ordering_check.h` in libc++ sources.