Handle Ada extended access thick pointers

Handle Ada extended access thick pointers

In Ada, sometimes an array is represented as a "thick" pointer -- a
structure that holds a pointer to the array data and another pointer
to the bounds structure.

A new "extended access" feature is being added to GNAT which changes
the shape of these objects.  With the new feature, the bounds are
inlined into the thick pointer.

This patch changes gdb to understand this new feature.  A test case is
provided; it is written in C to avoid requiring a newer GNAT just for
this test.

Approved-By: Andrew Burgess <aburgess@redhat.com>