[Ada] Array bound incorrectly printed for array indexed by enum subrange

[Ada] Array bound incorrectly printed for array indexed by enum subrange

Consider the following declarations:

   type Range_Type is (One, Two, Three);
   type Array_Type is array (Range_Type range One .. Two) of Integer;
   A : Array_Type := (1, 2);

Trying to print A can yield:

   (gdb) print a
   $1 = (one => 1, 2)

The bound of the first element should not have been printed, since
"one" is the first enumerate of type Range_Type. Similarly, with
the following declarations:

   type Array2_Type is array (Range_Type range Two .. Three) of Integer;
   A2 : Array2_Type := (2, 3);

GDB is failing to print the bound of the first element of "A2":

   (gdb) print a2
   $2 = (2, 3)

This is because the index type for both types Array_Type and Array2_Type
are subranges (by DWARF definition for arrays), of an anonymous subrange
type. When deciding whether to print the bound of the first element,
we handle subranges, but only up to one level. This patch enhanced
the code to handle any number of subrange levels.

gdb/ChangeLog:

        * ada-valprint.c (print_optional_low_bound): Get index_type's
        target type for as long as it is a TYPE_CODE_RANGE.

No testcase with this patch, but this will be tested via the testcase
of another patch, which uses the DWARF assembler to generate debugging
info for an array indexed by an enum.