[Ada] Fix the evaluation of access to packed array subscript

[Ada] Fix the evaluation of access to packed array subscript

This change is relevant only for standard DWARF (as opposed to the GNAT
encodings extensions): at the time of writing it only makes a difference
with GCC patches that are to be integrated: see in particular
<https://gcc.gnu.org/ml/gcc-patches/2015-07/msg01364.html>.

Given the following Ada declarations:

    type Small is mod 2 ** 6;
    type Array_Type is array (0 .. 9) of Small
       with Pack;
    type Array_Access is access all Array_Type;

    A  : aliased Array_Type := (1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
    AA : constant Array_Type := A'Access;

Before this change, we would get the following GDB session:

    (gdb) print aa.all(2)
    $1 = 3
    (gdb) print aa(2)
    $2 = 16

This is wrong: both expression should yield the same value: 3.  The
problem is simply that the routine which handles accesses to arrays lack
general handling for packed arrays.  After this patch, we have the
expected output:

    (gdb) print aa.all(2)
    $1 = 3
    (gdb) print aa(2)
    $2 = 3

gdb/ChangeLog:

	* ada-lang.c (ada_value_ptr_subscript): Update the heading
	comment.  Handle packed arrays.

gdb/testsuite/ChangeLog:

	* gdb.ada/access_to_packed_array.exp: New testcase.
	* gdb.ada/access_to_packed_array/foo.adb: New file.
	* gdb.ada/access_to_packed_array/pack.adb: New file.
	* gdb.ada/access_to_packed_array/pack.ads: New file.

Tested on x86_64-linux, no regression.