Add option to dump CFGs in (simple) graphviz format during all passes.

Summary:
I noticed the BinaryFunction::viewGraph() method that hadn't been implemented
and decided I could use a simple DOT dumper for CFGs while working on the indirect
call optimization.

I've implemented the bare minimum for the dumper.  It's just nodes+BB labels with
dges. We can add more detailed information as needed/desired.

(cherry picked from FBD3509326)