[Bug libstdc++/94032] Please provide std::string::__resize_default_init

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94032

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
We'll add it when it's in the draft standard. Adding it before then would
either mean only making it available for C++20 mode, or adding it to the shared
library exports as a stable ABI artefact. Neither seems very good to me.