HFSort/call graph refactoring

Summary:
I've factored out the call graph code from dataflow and function reordering code and done a few small renames/cleanups.  I've also moved the function reordering pass into a separate file because it was starting to get big.

I've got more refactoring planned for hfsort/call graph but this is a start.

(cherry picked from FBD5140771)