[Bug libstdc++/106749] Implement C++23 library features

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106749

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Much already done in gcc-11 and gcc-12:
https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2023

See also
https://en.cppreference.com/w/cpp/compiler_support#C.2B.2B23_library_features