Improve Ada unchecked union type printing

Currently, "ptype" of an Ada unchecked union may show a
compiler-generated wrapper structure in its output.  It's more
Ada-like to elide this structure, which is what this patch implements.
It turned out to be simplest to reuse a part of print_variant_clauses
for this.

As this is Ada-specific, and Joel already reviewed it internally, I am
going to check it in.