Move interface expansion in hierarchy.cc into a helper class

Move interface expansion in hierarchy.cc into a helper class

There should be no functional change, but this splits up the control
flow across functions, using class fields to hold the state that's
being tracked. The result should be a bit easier to read.

This is part of work to add bind support, but I'm doing some
refactoring in the hierarchy pass to make the code a bit easier to
work with. The idea is that (eventually) the IFExpander object will
hold all the logic for expanding interfaces, and then other code can
do bind insertion.