[Ada] Ignore __XA types when redundant.

[Ada] Ignore __XA types when redundant.

Consider the following code which declares a variable A2 which
is an array of arrays of integers.

   type Array2_First is array (24 .. 26) of Integer;
   type Array2_Second is array (1 .. 2) of Array2_First;
   A1 : Array1_Second := ((10, 11, 12), (13, 14, 15));

Trying to print the type of that variable currently yields:

    (gdb) ptype A2
    type = array (1 .. 2, 24 .. 26) of integer

This is not correct, as this is the description of a two-dimension
array, which is different from an array of arrays. The expected
output is:

    (gdb) ptype a2
    type = array (1 .. 2) of foo_n926_029.array2_first

GDB's struct type currently handles multi-dimension arrays the same
way arrays of arrays, where each dimension is stored as a sub-array.
The ada-valprint module considers that consecutive array layers
are in fact multi-dimension arrays. For array of arrays, a typedef
layer is introduced between the two arrays, creating a break between
each array type.

In our situation, A2 is a described as a typedef of an array type...

        .uleb128 0x8    # (DIE (0x125) DW_TAG_variable)
        .ascii "a2\0"   # DW_AT_name
        .long   0xfc    # DW_AT_type

        .uleb128 0x4    # (DIE (0xfc) DW_TAG_typedef)
        .long   .LASF5  # DW_AT_name: "foo__array2_second"
        .long   0x107   # DW_AT_type

        .uleb128 0x5    # (DIE (0x107) DW_TAG_array_type)
        .long   .LASF5  # DW_AT_name: "foo__array2_second"
        .long   0xb4    # DW_AT_type
        .uleb128 0x6    # (DIE (0x114) DW_TAG_subrange_type)
        .long   0x11b   # DW_AT_type
        .byte   0x2     # DW_AT_upper_bound
        .byte   0       # end of children of DIE 0x107

... whose element type is, as expected, a typedef to the sub-array
type:

        .uleb128 0x4    # (DIE (0xb4) DW_TAG_typedef)
        .long   .LASF4  # DW_AT_name: "foo__array2_first"
        .long   0xbf    # DW_AT_type

        .uleb128 0x9    # (DIE (0xbf) DW_TAG_array_type)
        .long   .LASF4  # DW_AT_name: "foo__array2_first"
        .long   0xd8    # DW_AT_GNAT_descriptive_type
        .long   0x1c5   # DW_AT_type
        .uleb128 0xa    # (DIE (0xd0) DW_TAG_subrange_type)
        .long   0xf0    # DW_AT_type
        .byte   0x18    # DW_AT_lower_bound
        .byte   0x1a    # DW_AT_upper_bound
        .byte   0       # end of children of DIE 0xbf

The reason why things fails is that, during expression evaluation,
GDB tries to "fix" A1's type. Because the sub-array has a parallel
(descriptive) type (DIE 0xd8), GDB thinks that our array's index
type must be dynamic and therefore needs to be fixed. This in turn
causes the sub-array to be "fixed", which itself results in the
typedef layer to be stripped.

However, looking closer at the parallel type, we see...

        .uleb128 0xb    # (DIE (0xd8) DW_TAG_structure_type)
        .long   .LASF8  # DW_AT_name: "foo__array2_first___XA"
        [...]
        .uleb128 0xc    # (DIE (0xe4) DW_TAG_member)
        .long   .LASF10 # DW_AT_name: "foo__Tarray2_firstD1___XDLU_24__26"

... that all it tells us is that the array bounds are 24 and 26,
which is already correctly provided by the array's DW_TAG_subrange_type
bounds, meaning that this parallel type is just redundant.

Parallel types in general are slowly being removed in favor of
standard DWARF constructs. But in the meantime, this patch kills
two birds with one stone:

  1. It recognizes this situation where the XA type is useless,
     and saves an unnecessary range-type fixing;

  2. It fixes the issue at hand because ignoring the XA type results
     in no type fixing being required, which allows the typedef layer
     to be preserved.

gdb/ChangeLog:

        * ada-lang.c (ada_is_redundant_range_encoding): New function.
        (ada_is_redundant_index_type_desc): New function.
        (to_fixed_array_type): Ignore parallel XA type if redundant.

gdb/testsuite/ChangeLog:

        * gdb.ada/arr_arr: New testcase.

Tested on x86_64-linux.