[Bug libstdc++/93121] std::bit_cast missing

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93121

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Not really a bug - C++20 isn't final yet, and we don't claim full support for
the current draft (not even close).