[analyzer] RetainCountChecker: refactor annotation handling.

...and add a new test case.

I thought this was broken, but it isn't; refactoring and reformatting anyway
so that I don't make the same mistake again. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178799 91177308-0d34-0410-b5e6-96231b3b80d8