Fix PR8625 and correctly interpret member-calls to static members when

Fix PR8625 and correctly interpret member-calls to static members when
producing warnings.

This feels really fragile, and I've not audited all other argument index-based
warnings. I suspect we'll grow this bug on another warning eventually. It might
be nice to adjust the argument indices when building up the attribute AST node,
as we already have to remember about the 'this' argument within that code to
produce correct errors.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@119340 91177308-0d34-0410-b5e6-96231b3b80d8