r222568 - Implement -Wcast-qual, fixing #13772.

[Roman Divacky]

> > +def warn_cast_qual2 : Warning<"cast from %0 to %1 must have all
> > intermediate "
> > +  "pointers const qualified to be safe">, InGroup<CastQual>,
> > DefaultIgnore;
> >
> 
> I think this is overstating the point; there are safe casts that do not
> have 'const' added in all intermediate points. It's a bit awkward, but we
> could use something like this:
> 
> "cast from pointer to %0 to pointer to non-const type %1 is not
> const-correct; storing a value through the resulting pointer may make the
> original pointer point to an object that is not of type %0"
 
Well, I modelled the warning on gcc, which warns like this:

warning: to be safe all intermediate pointers in cast from 'int **' to 'const
int **' must be 'const' qualified [-Wcast-qual]

What you suggest is a bit too long and I personally think that most people
will have to google what it means anyway. 

Maybe we can just keep my original warning text but just change "have" to
"should" ?

Opinions?

> Also, can you give this diagnostic a better name, like
> warn_cast_qual_indirect or warn_cast_qual_non_const_intermediate?

Sure.