Introduce die_info::children and use it

This adds a new die_info::children method.  This returns a range that
can be used to iterate over a DIE's children.

Then this goes through and updates all the relevant loops to use
foreach instead.  This is a net code reduction.

You'll note that in some places the code was checking the tag as well,
like:

      while (child_die && child_die->tag)

I believe this can't happen and is just a copy-paste oddity from the
old days.

Approved-By: Simon Marchi <simon.marchi@efficios.com>