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

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88322

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
C+2a status is now documented at
https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2020