[analyzer] Fix a crash on tracking Objective-C 'self' as a control dependency.

'self' was previously never tracked, but now it can be tracked
because it may be part of a condition.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@375328 91177308-0d34-0410-b5e6-96231b3b80d8