[LCG] Add some accessor methods to the SCC to allow iterating over the
parents of an SCC, and add a lookup method for finding the SCC for
a given function. These aren't used yet, but will be used shortly in
some unit tests I'm adding and are really part of the broader intended
interface for the analysis.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206959 91177308-0d34-0410-b5e6-96231b3b80d8