[Ada] print packed arrays indexed by enumerated type

[Ada] print packed arrays indexed by enumerated type

Consider the following declarations (a packed array indexed by an
enumerated type):

    type Color is (Black, Red, Green, Blue, White);
    type Full_Table is array (Color) of Boolean;
    pragma Pack (Full_Table);
    Full : Full_Table := (False, True, False, True, False);

GDB is unable to print the index values correctly. It prints the
enumeration's underlying value instead of the enumeration name:

    (gdb) p full
    $1 = (0 => false, true, false, true, false)
    (gdb) p full'first
    $2 = 0

And yet, it is capable of printing the correct type description:

    (gdb) ptype full
    type = array (black .. white) of boolean <packed: 1-bit elements>

To get to the real index type, one has to follow the parallel XA type.
We already do this for normal arrays. We can do it for this packed
array as well.

gdb/ChangeLog:

        * ada-lang.c (constrained_packed_array_type): If there is a
        parallel XA type, use it to determine the array index type.

gdb/testsuite/ChangeLog:

        * gdb.ada/arrayidx.exp: Adjust expected output for p_one_two_three.
        * gdb.ada/enum_idx_packed: New testcase.