(Ada) Fix Length attribute on array access

Consider the following variable "Indexed_By_Enum", declared as
an access to an array whose index type is an enumerated type
whose underlying values have "gaps":

   type Enum_With_Gaps is (LIT0, LIT1, LIT2, LIT3, LIT4);
   for Enum_With_Gaps use (LIT0 => 3,
                           LIT1 => 5,
                           LIT2 => 8,
                           LIT3 => 13,
                           LIT4 => 21);
   for Enum_With_Gaps'size use 16;

   type MyWord is range 0 .. 16#FFFF# ;
   for MyWord'Size use 16;

   type AR is array (Enum_With_Gaps range <>) of MyWord;
   type AR_Access is access AR;

   Indexed_By_Enum : AR_Access :=
     new AR'(LIT1 => 1,  LIT2 => 43, LIT3 => 42, LIT4 => 41);

Trying to print the length (number of elements) of this array using
the 'Length attribute does not work:

    (gdb) print indexed_by_enum'length
    'POS only defined on discrete types

The problem occurs while trying to get the array's index type.
It was using TYPE_INDEX_TYPE for that. It does not work for Ada arrays
in general; use ada_index_type instead.

gdb/ChangeLog:

	* ada-lang.c (ada_array_length): Use ada_index_type instead of
	TYPE_INDEX_TYPE.

gdb/testsuite/ChangeLog:

        * gdb.ada/arr_acc_idx_w_gap: New testcase.

Tested on x86_64-linux.

gdb/ada-lang.c

@@ -3179,7 +3179,7 @@ ada_array_length (struct value *arr, int n)
     }
 
   arr_type = check_typedef (arr_type);
-  index_type = TYPE_INDEX_TYPE (arr_type);
+  index_type = ada_index_type (arr_type, n, "length");
   if (index_type != NULL)
     {
       struct type *base_type;