[analyzer][RetainCount] Tie diagnostics to osx.cocoa.RetainCount rather then...

[analyzer][RetainCount] Tie diagnostics to osx.cocoa.RetainCount rather then RetainCountBase, for the most part

Similarly to other patches of mine, I'm trying to uniformize the checker
interface so that dependency checkers don't emit diagnostics. The checker that
made me most anxious so far was definitely RetainCount, because it is definitely
impacted by backward compatibility concerns, and implements a checker hierarchy
that is a lot different to other examples of similar size. Also, I don't have
authority, nor expertise regarding ObjC related code, so I welcome any
objection/discussion!

Differential Revision: https://reviews.llvm.org/D78099