[Bug libstdc++/88322] Implement C++20 library features.

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88322
Bug 88322 depends on bug 97930, which changed state.

Bug 97930 Summary: pair is not a structural type
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97930

           What    |Removed                     |Added
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             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED