Handle indexing Ada arrays with enum indices

Handle indexing Ada arrays with enum indices

In Ada, like C, an enum can assign values to the constants.  However,
unlike C (or any other language supported by gdb), the enum type can
also be used as the range of an array.

In this case, the user's code references the enum constants, but the
compiler translates these to the position of the constant in the enum.
So for example one might write:

   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
     );

Then index an array like "array(LIT3)" -- but this will be the 4th
element in an array of 5 elements, not the 13th element in an array of
19 (assuming I did the math right) elements.

gdb supports this to some degree, with the only missing piece being
indexing into such an array.  This patch implements this missing
feature, and also fixes an existing bug, which is that in some
situations I believe gdb would mis-compute the resulting array's
length.

The approach taken here is to try to integrate this feature into the
core of gdb.  My view is that much of the Ada support should be better
integrated with gdb, rather than being "on the side".  This, I think,
would help avoid code duplication at least.  So, I try to take steps
toward this goal when possible.

Because other languages generally don't allow the user to specify the
index type of an array, I simply made the core of gdb unconditionally
apply discrete_position when computing the range of such an array.
This is a no-op for ordinary types, but applies the enum
value-to-position transformation for TYPE_CODE_ENUM.

gdb/ChangeLog
2020-05-26  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (ada_print_array_index): Change type.  Call val_atr.
	(ada_value_ptr_subscript): Don't call pos_atr on the lower bound.
	(val_atr): New function.
	(value_val_atr): Use it.
	* ada-valprint.c (print_optional_low_bound): Change low bound
	handling for enums.
	(val_print_packed_array_elements): Don't call discrete_position.
	* gdbtypes.c (get_discrete_bounds) <TYPE_CODE_RANGE>: Call
	discrete_position for enum types.
	* language.c (default_print_array_index): Change type.
	* language.h (struct language_defn) <la_print_array_index>: Add
	index_type parameter, change type of index_value.
	(LA_PRINT_ARRAY_INDEX): Add index_type parameter.
	(default_print_array_index): Update.
	* valprint.c (maybe_print_array_index): Don't call
	value_from_longest.  Update.
	(value_print_array_elements): Don't call discrete_position.

gdb/testsuite/ChangeLog
2020-05-26  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/arr_acc_idx_w_gap.exp: Add tests.