[libc++][NFC] Rename helper function for testing spaceship

The helper is mis-named, since it won't work as-is on ordered containers
like set and map, because they rely on being able to store keys that are
partial_ordering::unordered, and that's UB for an ordered container.

This was most likely a typo or an unintended naming mistake, since
the function is only used with sequence containers anyway.

Differential Revision: https://reviews.llvm.org/D146991