[objc-arc] Change 4 iterator methods which return const_iterators to be const methods.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187940 91177308-0d34-0410-b5e6-96231b3b80d8
[objc-arc] Track if we encountered an additive overflow while computing {TopDown,BottomUp}PathCounts and do nothing if it occured.

rdar://14590914

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187941 91177308-0d34-0410-b5e6-96231b3b80d8