[libc++][spaceship][NFC] P0905R1: Symmetry for spaceship - mark as implemented (#87871)

Implemented: https://wg21.link/P0905R1 in
0913ca19
- https://eel.is/c++draft/cmp.weakeq (Removed in
https://wg21.link/P1959R0)
- https://eel.is/c++draft/cmp.strongeq (Removed in
https://wg21.link/P1959R0)
- https://eel.is/c++draft/cmp.partialord
-
https://github.com/llvm/llvm-project/blob/4abb722ffa7fcf809faa4a479fdf2f78c685b351/libcxx/include/__compare/ordering.h#L101
- https://eel.is/c++draft/cmp.weakord
-
https://github.com/llvm/llvm-project/blob/4abb722ffa7fcf809faa4a479fdf2f78c685b351/libcxx/include/__compare/ordering.h#L173
- https://eel.is/c++draft/cmp.strongord
-
https://github.com/llvm/llvm-project/blob/4abb722ffa7fcf809faa4a479fdf2f78c685b351/libcxx/include/__compare/ordering.h#L250

References:
- https://wg21.link/P1959R0 - Remove `std::weak_equality` and
`std::strong_equality`