[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 #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I'm starting to think we should just make libstdc++ headers 100% warning-free
and stop marking them as system headers with the pragmas. When they're
installed under /usr they'd still be considered system headers though.