GH-50660: [C++][Dev] Add Decimal32 and Decimal64 GDB pretty-printers

[fenfeng9]

Rationale for this change

The Arrow GDB pretty-printers support Decimal128 and Decimal256, but do not support Decimal32 or Decimal64.

What changes are included in this PR?

• Add GDB pretty-printer support and tests for Decimal32 and Decimal64.
• Remove the unused max_type_id variable.

Are these changes tested?

Yes.

Are there any user-facing changes?

No.

• GitHub Issue: [C++][Dev] Implement Decimal32/64 support in gdb_arrow.py #50660

[pitrou]

@Reranko05 Do you feel like reviewing this?

[fenfeng9]

Here are some examples from a local GDB session.

// Scalars
Decimal32Scalar decimal32_scalar_pos{
    Decimal32("1234567"), decimal32(9, 4)};
Decimal64Scalar decimal64_scalar_pos{
    Decimal64("12345678901234567"), decimal64(18, 4)};

// Arrays
auto heap_decimal32_array =
    SliceArrayFromJSON(
        decimal32(9, 4),
        R"([null, "-12345.6789", "12345.6789"])");
auto heap_decimal64_array =
    SliceArrayFromJSON(
        decimal64(18, 4),
        R"([null, "-12345678901234.5678", "12345678901234.5678"])");
auto heap_decimal32_array_sliced =
    heap_decimal32_array->Slice(1, 1);

(gdb) p decimal32_scalar_pos
$1 = arrow::Decimal32Scalar of value 123.4567 [precision=9, scale=4]

(gdb) p decimal64_scalar_pos
$2 = arrow::Decimal64Scalar of value 1234567890123.4567 [precision=18, scale=4]

(gdb) p *heap_decimal32_array
$3 = (std::__shared_ptr_access<arrow::Array, (__gnu_cxx::_Lock_policy)2, false, false>::element_type &) @0x43aa50:
     arrow::Decimal32Array of type arrow::decimal32(9, 4),
     length 3, offset 0, null count 1 = {null, -12345.6789, 12345.6789}

(gdb) p *heap_decimal64_array
$4 = (std::__shared_ptr_access<arrow::Array, (__gnu_cxx::_Lock_policy)2, false, false>::element_type &) @0x43aaa0:
     arrow::Decimal64Array of type arrow::decimal64(18, 4),
     length 3, offset 0, null count 1 = {null, -12345678901234.5678, 12345678901234.5678}

(gdb) p *heap_decimal32_array_sliced
$5 = (std::__shared_ptr_access<arrow::Array, (__gnu_cxx::_Lock_policy)2, false, false>::element_type &) @0x43b060:
     arrow::Decimal32Array of type arrow::decimal32(9, 4),
     length 1, offset 1, unknown null count = {-12345.6789}

[Reranko05]

LGTM :)