[libc++] Fix missing requires clause on variant operator<=>

`std::variant::operator<=>` is missing a requires clause ensuring that
`operator<=>` only exists when all of the types in the variant are
`three_way_comparable`.

Add the missing requires clause and adjust the existing test which was
incorrect.

Fixes https://github.com/llvm/llvm-project/issues/58192.

Differential Revision: https://reviews.llvm.org/D136050