Improve self-comparison debug mode

Improve self-comparison debug mode

When looking at debugging some self-comparison difference errors, I
felt the need to improve the debugging support for self-comparison
(triggered by 'abidw --debug-abidiff <binary>').

This patch fixes some typos in the existing diagnostics emitted by the
self-comparison debugging mode and improves the detection of a change
in a canonical type value between a type that is serialized to abixml,
and the same type that is de-serialized from abixml.  This later check
is now performed during the process of handling canonical type
propagation when confirming/canceling (speculatively) propagated
canonical types.  It's useful to find problems in that process of
confirming/canceling.

	* include/abg-ir.h
	(environment::{get_type_id_canonical_type_map}): Const-ify the
	existing member function and add non-const overloads.
	(environment::{get_type_id_from_pointer,
	get_canonical_type_from_type_id}): Const-ify.
	(environment::get_pointer_type_id_map): Add new member function.
	* src/abg-ir-priv.h
	(environment::priv::{confirm_ct_propagation_for_types_dependant_on,
	confirm_ct_propagation}): Call
	check_abixml_canonical_type_propagation_during_self_comp() here.
	(environment::priv::{get_type_id_canonical_type_map,
	get_pointer_type_id_map, get_type_id_from_pointer,
	get_type_id_from_type, get_canonical_type_from_type_id,
	check_abixml_canonical_type_propagation_during_self_comp}): Add
	new member functions.
	* src/abg-ir.cc (return_comparison_result): Call
	check_abixml_canonical_type_propagation_during_self_comp on every
	single type with non confirmed propagated canonical type.
	(environment::{get_type_id_canonical_type_map,
	get_pointer_type_id_map, get_type_id_from_pointer,
	get_type_id_from_type, get_canonical_type_from_type_id}): Delegate
	to the new implementations that are now member functions of
	environment::priv.
	(type_base::get_canonical_type_for): Fix typo in diagnostics when
	debugging self-comparison. Add more context.
	* src/abg-reader.cc
	(abixml::reader::maybe_check_abixml_canonical_type_stability):
	Likewise.
	* src/abg-writer.cc (write_type_record): Do not try to get abixml
	type-id for a type (originating from the DWARF) that has no
	canonical type, otherwise, that /can/ introduce a gap in the
	type-ids as those can turn out not being emitted in the abixml
	after all.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>