[Ada] 'first/'last/'length of array whose bound is a discriminant

[Ada] 'first/'last/'length of array whose bound is a discriminant

Consider the following code:

   type Table is array (Positive range <>) of Integer;
   type Object (N : Integer) is record
       Data : Table (1 .. N);
   end record;
   My_Object : Object := (N => 3, Data => (3, 5, 8));

Trying to print the range and length of the My_Object.Data array yields:

    (gdb) print my_object.data'first
    $1 = 1
    (gdb) print my_object.data'last
    $2 = 0
    (gdb) print my_object.data'length
    $3 = 0

The first one is correct, and that is thanks to the fact that
the lower bound is statically known.  However, for the upper
bound, and consequently the array's length, the values are incorrect.
It should be:

    (gdb) print my_object.data'last
    $2 = 3
    (gdb) print my_object.data'length
    $3 = 3

What happens here is that ada_array_bound_from_type sees that
our array has a parallel "___XA" type, and therefore tries to
use it.  In particular, it described our array's index type as:
[...]___XDLU_1__n, which means lower bound = 1, and upper bound
is value of "n". Unfortunately, ada_array_bound_from_type does
not have access to the discriminant, and is therefore unable to
compute the bound correctly.

Fortunately, at this stage, the bound has already been computed
a while ago, and therefore doesn't need to be re-computed here.
This patch fixes the issue by ignoring that ___XA type if the array
is marked as already fixed.

This also fixes the same issue with packed arrays.

gdb/ChangeLog:

        * ada-lang.c (ada_array_bound_from_type): Ignore array's parallel
        ___XA type if the array has already been fixed.

gdb/testsuite/ChangeLog:

        * gdb.ada/var_arr_attrs: New testcase.