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

[david at doublewise dot net]

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

--- Comment #3 from David Stone <david at doublewise dot net> ---
Also filed against libc++ here: https://llvm.org/bugs/show_bug.cgi?id=28777