[objc-arc] Track if we encountered an additive overflow while computing...

[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

llvm-svn: 188122