[Bug libstdc++/110339] Implement C++26 library features

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110339
Bug 110339 depends on bug 121748, which changed state.

Bug 121748 Summary: [C++26] Implement P2988R12, std::optional<T&>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121748

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