Array indexed by non-contiguous enumeration types

Array indexed by non-contiguous enumeration types

In Ada, index types of arrays can be enumeration types, and enumeration
types can be non-contiguous. In which case the address of elements is
not given by the value of the index, but by its position in the enumeration
type.

In other words, in this example:

 type Color is (Blue, Red);
 for Color use (Blue => 8, Red => 12, Green => 16);

 type A is array (Color) of Integer;
 type B is array (1 .. 3) of Integer;

Arrays of type A and B will have the same layout in memory, even if
the enumeration Color has a hole in its set of integer value.

Since recently support for such a feature was in ada-lang.c, where the
array was casted to a regular continuous index range. We were losing
the information of index type. And this was not quite working for
subranges in variable-length fields; their bounds are expressed using
the integer value of the bounds, not its position in the enumeration,
and there was some confusion all over ada-lang.c as to whether we had
the position or the integer value was used for indexes.

The idea behind this patch is to clean this up by keeping the real
representation of these array index types and bounds when representing
the value, and only use the position when accessing the elements or
computing the length. This first patch fixes the printing of such
an array.

To the best of my knowledge, this feature only exists in Ada so it
should only affect this language.

gdb/ChangeLog:

        Jerome Guitton  <guitton@adacore.com>:
        * ada-lang.c (ada_value_ptr_subscript): Use enum position of
        index to get element instead of enum value.
        (ada_value_slice_from_ptr, ada_value_slice): Use enum position
        of index to compute length, but enum values to compute bounds.
        (ada_array_length): Use enum position of index instead of enum value.
        (pos_atr): Move position computation to...
        (ada_evaluate_subexp): Use enum values to compute bounds.
        * gdbtypes.c (discrete_position): ...this new function.
        * gdbtypes.h (discrete_position): New function declaration.
        * valprint.c (val_print_array_elements): Call discrete_position
        to handle array indexed by non-contiguous enumeration types.

gdb/testsuite/ChangeLog:

        * gdb.ada/arr_enum_with_gap: New testcase.