The corresponding `std` types were removed before C++20 was finalized:
https://wg21.link/P1959R0.
They are unused. The language mechanisms and recommendations changed since they
were originally proposed. In particular:
* An explicitly defaulted `operator==` is defined in terms of memberwise
`operator==` rather than sibling `operator<=>`.
* An `operator!=` can be implicitly added to an overload set in terms of
`operator==` rather than `operator<=>`.
* A class which has equality but not ordering defined should provide
`operator==` rather than `operator<=>`.
PiperOrigin-RevId: 587834267
Change-Id: I2c2513c13f3485b9edc6a345dca4a577d8e65167
