[analyzer] More reliably detect property accessors.

This has a side effect of preventing a crash, which occurs because we get a
property getter declaration, which is overriding but is declared inside
@protocol. Will file a bug about this inconsistency internally. Getting a
small test case is very challenging.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@190836 91177308-0d34-0410-b5e6-96231b3b80d8