tweak warning options to be more like gcc:

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@70362 91177308-0d34-0410-b5e6-96231b3b80d8