[Bug libstdc++/88339] Implement P0515R3, C++20 three-way comparison operator support .

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88339

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yeah, I already noticed that :-)

I have some more changes locally too.