Implement the newest status quo for method override checking. The idea now

is that we need more information to decide the exact conditions for whether
one ObjCObjectPointer is an acceptable return/parameter override for another,
so we're going to disable that entire class of warning for now.  The
"forward developement" warning category, -Wmethod-signatures, can receive
unrestricted feature work, and when we're happy with how it acts, we'll
turn it on by default.

This is a pretty conservative change, and nobody's totally content with it.

llvm-svn: 117524