[C++] Simplify IPC writer for dense unions

[asfimport]

In ARROW-10580 we fixed the Arrow C++ implementation so that dense union offsets are always (non-strictly) monotonic for a given child, as mandated by the spec.

The IPC writer implementation, however, still assumes that dense union offsets may be in any order:

arrow/cpp/src/arrow/ipc/writer.cc

Lines 476 to 485 in 5719576

	// Offsets may not be ascending, so we need to find out the start offset
	// for each child
	for (int64_t i = 0; i < length; ++i) {
	const uint8_t code = type_codes[i];
	if (child_offsets[code] == -1) {
	child_offsets[code] = unshifted_offsets[i];
	} else {
	child_offsets[code] = std::min(child_offsets[code], unshifted_offsets[i]);
	}
	}

This can probably be simplified, making it slightly less costly to emit a sliced union array.

Reporter: Antoine Pitrou / @pitrou

Related issues:

• [C++] When Validating, ensure DenseUnionArray offsets are increasing (relates to)

_{Note: This issue was originally created as ARROW-17339. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Also for the record the Go code for this is as follows:

	for i, c := range codes {
		if offsets[c] == -1 {
			// offsets are guaranteed to be increasing according to the spec
			// so the first offset we find for a child is the initial offset
			// and will become the "0" for this child.
			offsets[c] = unshiftedOffsets[i]
			shiftedOffsets[i] = 0
		} else {
			shiftedOffsets[i] = unshiftedOffsets[i] - offsets[c]
		}
		lengths[c] = maxI32(lengths[c], shiftedOffsets[i]+1)
	}

[rtadepalli]

Hi, I'd like to work on this if it is still available. Thanks.