[LCG] Hoist the logic for forming a new SCC from the top of the DFSStack
into a helper function. I plan to re-use it for doing incremental
DFS-based updates to the SCCs when we mutate the call graph.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206948 91177308-0d34-0410-b5e6-96231b3b80d8