[LCG] Switch the SCC lookup to be in terms of call graph nodes rather
than functions. So far, this access pattern is *much* more common. It
seems likely that any user of this interface is going to have nodes at
the point that they are querying the SCCs.

No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207045 91177308-0d34-0410-b5e6-96231b3b80d8