[Bug libstdc++/114153] std::less<> prefers operator const void*() over operator<=>() in C++20 mode

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114153

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Without looking at the code, we probably just need to check if the type has a
usable operator<=>.

We check it out had a usable operator< which worked fine in C++17, but in C++20
x<y can work without having any operator<