[RFA] Introdue warning_n; fix singulars in the final keyword wanrings

[Joseph S. Myers]

On Sun, 3 Aug 2014, Jan Hubicka wrote:

> 	* diagnostic.c (warning_n): New function.
> 	* diagnostic-core.h (warning_n): Declare.
> 	* ipa-devirt.c (ipa_devirt): Handle singulars correctly;
> 	output dynamic counts when available.

OK, though note the pre-existing issue that these diagnostics start with 
uppercase letters, contrary to GNU style.

-- 
Joseph S. Myers
joseph@codesourcery.com