varobj/Ada: Missing children for interface-wide tagged types

varobj/Ada: Missing children for interface-wide tagged types

Consider the following code:

   type Element is abstract tagged null record;
   type GADataType is interface;
   type Data_Type is new Element and GADataType with record
      I : Integer := 42;
   end record;
   Result1 : Data_Type;
   GGG1    : GADataType'Class := GADataType'Class (Result1);

When trying to create a varobj for variable ggg1, GDB currently
returns an object which has no child:

    -var-create ggg1 * ggg1
    ^done,name="ggg1",numchild="0",[...]

This is incorrect, it should return an object which has one child
(field "i"). This is because tagged-type objects are dynamic, and
we need to apply a small transformation in order to get their actual
type. This is already done on the GDB/CLI side in ada-valprint,
and it needs to be done on the ada-varobj side as well.

gdb/ChangeLog:

        * ada-varobj.c (ada_varobj_adjust_for_child_access): Convert
        tagged type objects to their actual type.

gdb/testsuite/ChangeLog:

        * gdb.ada/mi_interface: New testcase.