[libcxx] removes operator!= and globally guards against no spaceship operator

[cjdb]

Summary

• operator!= isn't in the spec
• <compare> is designed to work with operator<=> so it doesn't really make sense to have operator<=>-less friendly sections.

Depends on D100283.

[ldionne]

LGTM. I assume we didn't have tests that would need changing?

[• Quuxplusone]

This seems like a step in the right direction. Would you be interested in jumping all the way to the full C++20-mandated hidden-friend implementation in one fell swoop? Or is that already on your roadmap?

cppreference says:

These functions are not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when std::partial_ordering [or whatever] is an associated class of the arguments.

In other words, they should be defined inline, as hidden friends.

[cjdb]

rebases to activate CI

[cjdb]

rebases to activate CI

[cjdb]

adds // UNSUPPORTED: apple-clang-9, apple-clang-10, apple-clang-11, apple-clang-12.0.0 to the files failing CI (AppleClang <12.0.1 seems to not work)