Use the argument location instead of the format string location when warning

about argument type mismatch.

This gives a nicer diagnostic in cases like
  printf(fmt,
         i);
where previously the snippet just pointed at 'fmt' (with a note at the
definition of fmt).

It's a wash for cases like
  printf("%f",
         i);
where previously we snippeted the offending portion of the format string,
but didn't indicate which argument was at fault.

llvm-svn: 156968