Support incomplete enums in core and diff code.

Support incomplete enums in core and diff code.

This is an initial implementation of the support for incomplete, also
known as forward-declared, enum types. I've not made any attempt to
refactor or share logic with the struct/union code.

	* include/abg-comp-filter.h (has_decl_only_def_change) : Declare
	New function.
	* src/abg-comp-filter.cc (there_is_a_decl_only_enum): Define new
	static function and ...
	(type_size_changed): ... use it here.
	(has_decl_only_def_change): Define new function and ...
	(categorize_harm{less, ful}_diff_node): ... use it here.
	* include/abg-fwd.h (enums_type, decl_base_wptr): Declare new
	typedefs.
	(look_through_decl_only_class): Declare new overload for
	class_or_union*.
	(is_compatible_with_enum_type, is_compatible_with_enum_type)
	(look_through_decl_only, lookup_enum_types, lookup_enum_types):
	Declare new functions.
	* include/abg-ir.h (decl_base::{get_is_declaration_only,
	set_is_declaration_only, set_definition_of_declaration,
	get_definition_of_declaration,
	get_naked_definition_of_declaration}): Declare new member
	functions.  They were moved here from the class_or_union class.
	(class_or_union::{get_earlier_declaration,
	set_earlier_declaration, get_definition_of_declaration,
	set_definition_of_declaration,
	get_naked_definition_of_declaration, get_is_declaration_only,
	set_is_declaration_only}): Remove these member functions.
	* src/abg-ir.cc (decl_base::priv::{declaration_,
	definition_of_declaration_, naked_definition_of_declaration_,
	is_declaration_only_}): Define data members.  Moved here from
	class_or_union.
	(decl_base::priv::priv): Adjust to initialize the new data
	members.
	(decl_base::{get_earlier_declaration, set_earlier_declaration,
	get_definition_of_declaration,
	get_naked_definition_of_declaration, get_is_declaration_only,
	set_is_declaration_only, set_definition_of_declaration}): Define
	member functions.
	(operator|): In the overload for (change_kind, change_kind),
	adjust the return type of the call to
	decl_base::get_definition_of_declaration.
	(look_through_decl_only): Define new function.
	(look_through_decl_only_class): Adjust.
	(look_through_decl_only_enum): Likewise.
	(maybe_update_types_lookup_map<class_decl>): Adjust return type of
	call to decl_base::get_definition_of_declaration.
	(types_defined_same_linux_kernel_corpus_public): Use
	look_through_decl_only_class rather than open coding it.
	(class_or_union::priv::{declaration_, definition_of_declaration_,
	naked_definition_of_declaration_, is_declaration_only_}): Remove
	these data members.  They are now carried by decl_base::priv.
	(class_or_union::{g,s}et_alignment_in_bits): Adjust.
	(class_or_union::{g,s}et_size_in_bits): Likewise.
	(class_or_union::operator==): Likewise.
	(equals): Adjust the overload for class_or_union.
	(is_compatible_with_enum_type)
	* src/abg-comparison.cc (try_to_diff<class_decl>): Adjust the
	return type of decl_base::get_definition_of_declaration.
	(leaf_diff_node_marker_visitor::visit_begin): Use
	filtering::has_decl_only_def_change rather than
	filtering::has_class_decl_only_def_change.  Decl-only changes to
	enums (or any other type really) will thus not be recorded as leaf
	changes.
	* src/abg-dwarf-reader.cc (get_scope_for_die): Adjust return type
	of decl_base::get_definition_of_declaration.
	* src/abg-default-reporter.cc (default_reporter::report): Report
	enum decl-only <-> definition changes.
	* src/abg-hash.cc (class_or_union::hash::operator()): In the
	overload for class_or_union& adjust the return type for
	decl_base::get_definition_of_declaration.

Signed-off-by: Giuliano Procida <gprocida@google.com>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>