[CFG] Set the “loop target” (back edge) for VisitObjCForCollectionStmt loops

Add the back edge info by creating a basic block, marked as loop target. This is
consistent with how other loops are processed, but was omitted from
VisitObjCForCollectionStmt.
[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.