Add a basic CallGraph to Analysis.

The final graph contains a single root node, which is a parent of all externally available functions(and 'main'). As well as a list of Parentless/Unreachable functions, which are either truly unreachable or are unreachable due to our analyses imprecision.

The analyzer checkers debug.DumpCallGraph or debug.ViewGraph can be used to look at the produced graph.

Currently, the graph is not very precise, for example, it entirely skips edges resulted from ObjC method calls.

llvm-svn: 152272