C/C++ PATCH to implement -Wpointer-compare warning (PR c++/64767)

[Martin Sebor]

> +		permerror (input_location, "ISO C++11 only allows pointer "
> +			   "conversions for integer literals");

FWIW, I think it would be clearer to either mention the currently
selected language version or leave it out completely rather than
hardcoding C++11.  When the user specifies -std=c++14 (or later),
or when that's the current version used by the compiler, a warning
that tells them what C++ 11 allows isn't really relevant (or
meaningful), and becomes less so as time goes on.

Martin