[Bug libstdc++/93590] [diagnostic] The header is deprecated since C++17, but no warning for #include-ing it

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93590

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
There's no replacement for <codecvt> yet.