[analyzer] Use output form collections’ count to decide if ObjC for loop should be...

[analyzer] Use output form collections’ count to decide if ObjC for loop should be entered

This fixes false positives by allowing us to know that a loop is always entered if
the collection count method returns a positive value and vice versa.

Addresses radar://14169391.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184618 91177308-0d34-0410-b5e6-96231b3b80d8