(Ada) change tagged types base_address computation

(Ada) change tagged types base_address computation

There was a difference between C++ dispatch table and Ada's in the
way the Offset_To_Top field is used to determined the base address
of an object:

* in C++ it is a negative offset, so converting abstract interface to
deriving object requires adding this offset to “this”;

* in Ada, it was a positive offset, so the same conversion required
subtracting the offset value.

So in ada, the base address for a tagged type was computed using this formula:

base_address = value_address (obj) - offset_to_top;

The offset_to_top value was previously set to 0 or a positive value.
With recent version of AdaCore's GNAT compiler, the offset has been
changed to match C++, which means it's set to zero or a negative value

As a result, the new formula has to be:

base_address = value_address (obj) + offset_to_top;

Because we want to support old code compiled before GNAT compiler change
done in 19.0w (20171023-64) with this version and future versions of gdb,
then we change the sign of the offset_to_top if required. Required here
means if offset_to_top is positive since it indicates that the code has
been compiled with an old GNAT compiler.

This patch changes the formula as described above.

Also, one side-effect of offset_to_top now being negative is that
we now have to worry about the sign when we read its value from the
inferior. Up to now, we have been reading its value using the data
address builtin type. But since addresses are not always signed, we
now need to make sure we use the proper type (type Storage_Offset
from System.Storage_Elements). Ideally, we would be looking this type
up from the inferior, and then use that type. However, it is not
guaranteed that this type always be described in the debugging
information, so this patch just builds our own, adding it to Ada's
list of primitive types.

gdb/ChangeLog:

        * ada-lang.c (ada_tag_value_at_base_address): Change the way
        tagged type base address is computed.
        (enum ada_primitive_types) <ada_primitive_type_storage_offset>:
        New enumerate.
        (ada_language_arch_info): Set the ada_primitive_type_storage_offset
        element of lai->primitive_type_vector.

Tested on x86_64-linux. Fixes the following tests when using the newer
version of the compiler.

    gdb.ada/iwide.exp: print My_Drawable
    gdb.ada/iwide.exp: print d_access.all
    gdb.ada/iwide.exp: print dp_access.all
    gdb.ada/mi_interface.exp: create ggg1 varobj (unexpected output)
    gdb.ada/mi_interface.exp: list ggg1's children (unexpected output)
    gdb.mi/mi-var-rtti.exp: run to mi-var-rtti.cc:63 (set breakpoint) (unexpected output)
    gdb.mi/mi-var-rtti.exp: run to mi-var-rtti.cc:63 (set breakpoint)