introduce a new CallGraphSCC class, and pass it around

introduce a new CallGraphSCC class, and pass it around
to CallGraphSCCPass's instead of passing around a
std::vector<CallGraphNode*>.  No functionality change,
but now we have a much tidier interface.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@101558 91177308-0d34-0410-b5e6-96231b3b80d8