[StaticAnalysis] Remove unnecessary parameter in CallGraphNode::addCallee.

Summary:
Remove the CallGraph in addCallee as it is not used in addCallee.
It decouples addCallee from CallGraph, so that we can use CallGraphNode
within our customized CallGraph.

Reviewers: bkramer

Subscribers: cfe-commits, ioeric

Differential Revision: https://reviews.llvm.org/D27674

llvm-svn: 289431