[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 #13 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #10)
> I'm starting to think we should just make libstdc++ headers 100%
> warning-free

We're closer to that now, but not in a position to stop marking our headers as
system headers.