[libc++][spaceship] Implement std::variant::operator<=>

Implements [variant.relops] and [variant.monostate.relops] for P1614R2

Reviewed By: Mordante, #libc, avogelsgesang

Differential Revision: https://reviews.llvm.org/D131372

GitOrigin-RevId: c4566cac490cf81c141e7deb84008b2cc1de48b7
6 files changed