Decode "dynamic" interface types in Ada

Decode "dynamic" interface types in Ada

In Ada, if a class implements an interface and has a dynamic
superclass, then the "offset to top" -- the offset that says how to
turn a pointer to the interface into a pointer to the whole object --
is stored in the object itself.  This patch changes GDB to understand
this.

Because this only touches Ada code, and because Joel already reviewed
it internally, I am checking it in.