Bug 19355 - Libabigail slow on r300_dri.so

Bug 19355 - Libabigail slow on r300_dri.so

Profiling abidw --abidiff r300_dri.so, with a r30_dri.so that was run
through dwz shows that a great deal of the time is spent in trying to
get the logical parent of a DIE which has been imported.

To do that, we need to walk the translation unit DIEs again to know
where the DIE we are looking at has been imported.  And doing that
walking again and again, following the accessors of the DIE data
structure from elfutils takes time.

This patch reduces that time by constructing a "trace" of where units
have been imported.  So that looking for the logical parent of a given
DIE doesn't involve walking the DIE tree itself, but rather, walking
the trace, which is a vector.  This proves to be much faster.

In practice, the overall time spent is now less than 12 minutes.  It
was more than 50 minutes before.

	* src/abg-dwarf-reader.cc (struct imported_unit_point): Define new
	type.
	(operator<(const imported_unit_point&, const
	imported_unit_point&)): Define less-than operator for new
	imported_unit_point& type.
	(imported_unit_points_type, tu_die_imported_unit_points_map_type):
	New typedefs.
	(find_lower_bound_in_imported_unit_points): Define new static function.
	(read_context::tu_die_imported_unit_points_map_): New data member.
	(read_context::tu_die_imported_unit_points_map): New getter.
	(die_die_attribute): Define new overload.
	(build_die_parent_relations_under): Take imported_unit_points_type
	output parameter and populate it along the way. Remove the
	overload that takes a read_context as a parameter.
	(build_primary_die_parent_relations_under)
	(build_alternate_die_parent_relations_under): Remove.
	(build_die_parent_maps): Pass an instance of
	imported_unit_points_type to build_die_parent_relations_under.
	(find_import_unit_point_between_dies): Rename one overload of
	find_last_import_unit_point_before_die into this.  Adjust to make
	it find the import point between two offsets.
	(find_import_unit_point_before_die): Rename the other overload of
	find_last_import_unit_point_before_die into this. Adjust to use
	find_import_unit_point_between_dies.
	(get_parent_die): Adjust to use find_import_unit_point_before_die.

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