[ObjCARC Debug Messages] This is a squashed commit of 3x debug message commits ala echristo's suggestion.

1. Added debug messages when in OptimizeIndividualCalls we move calls into predecessors and then erase the original call.
2. Added debug messages when in the process of moving calls in ObjCARCOpt::MoveCalls we create new RR and delete old RR.
3. Added a debug message when we visit a specific retain instruction in ObjCARCOpt::PerformCodePlacement.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171988 91177308-0d34-0410-b5e6-96231b3b80d8
Move the internal PrintStackTrace function that is used for llvm::sys::PrintStackTraceOnErrorSignal(),
into a new function llvm::sys::PrintStackTrace, so that it's available to clients for logging purposes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171989 91177308-0d34-0410-b5e6-96231b3b80d8