C PATCH to further improve diagnostic for -Wsign-compare (PR c/81417)

[Joseph Myers]

On Wed, 2 Aug 2017, Marek Polacek wrote:

> Hmm, how about this, then?
> 
> "operand of ?: changes signedness from %qT to %qT due to unsignedness of other operand"
> 
> I couldn't come up with anything more brief yet conveying all the information.
> I don't like adding "second"/"third"/... very much; we should offer a good
> location already.
> 
> Bootstrapped/regtested on x86_64-linux, ok for trunk?

OK.

-- 
Joseph S. Myers
joseph@codesourcery.com