tweak warning options to be more like gcc:

1. All all variants of -Wformat*, make them imply -Wformat.  GCC warns 
   if you use -Wformatfoo without -Wformat.  We just make one imply the 
   other.
2. Make -Wformat-nonliteral default to off, like gcc.  It is an incredible
   nuisance.
3. Accept but currently ignore -Wformat-extra-args.

llvm-svn: 70362