This adds profile guided layout using the Call-Chain Clustering (C³) heuristic from https://research.fb.com/wp-content/uploads/2017/01/cgo2017-hfsort-final1.pdf .
RFC: [llvm-dev] [RFC] Profile guided section layout http://lists.llvm.org/pipermail/llvm-dev/2017-June/114178.html
dead declaration.