[objc-arc] Track if we encountered an additive overflow while computing {TopDown,BottomUp}PathCounts and do nothing if it occurred.

I fixed the aforementioned problems that came up on some of the linux boxes.
Major thanks to Nick Lewycky for his help debugging!

rdar://14590914

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188122 91177308-0d34-0410-b5e6-96231b3b80d8