Re-work the handling of implicit 'this' arguments and silly GCC-style attribute

argument indexes. This handles the offsets in a consistent manner for all of
the attributes which I saw working with these concepts. I've also added tests
for the attribute that motivated this: nonnull.

I consolidated the tests for format attributes into one file, and fleshed them
out a bit to trigger more of the warning cases. Also improved the quality of
some of the diagnostics that occur with invalid argument indices.

The only really questionable change here is supporting the implicit this
argument for the ownership attribute. I'm not sure it's really a sensible
concept there, but implemented the logic for consistency.

llvm-svn: 119339