[Bug libstdc++/54924] Warn for std::string constructor with wrong size

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54924

--- Comment #12 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Indeed, it's what allows us to use variadic templates in C++98 mode, for
example. And I don't think there's any way to use
__attribute__((__extension__)) on template parameter packs to do that
differently.