Bug 24731 - Wrongly reporting union members order change

Bug 24731 - Wrongly reporting union members order change

When union data members are re-ordered, abidiff reports the
re-ordering as if it was a meaningful ABI change.

This patch teaches Libabigail to categorize that benign type layout
change as a HARMLESS_UNION_CHANGE_CATEGORY kind of change and ignore it.

	* include/abg-comp-filter.h (union_diff_has_harmless_changes):
	Declare new function and ...
	* src/abg-comp-filter.cc (union_diff_has_harmless_changes):
	... define it here.
	(categorize_harmless_diff_node): Use the new
	union_diff_has_harmless_changes here.
	* include/abg-comparison.h (HARMLESS_UNION_CHANGE_CATEGORY): Add a
	new enumerator to diff_category enum.  Adjust the value of the
	other enumerators.
	* src/abg-comparison.cc (get_default_harmless_categories_bitmap):
	Add the new HARMLESS_UNION_CHANGE_CATEGORY in here.
	(operator<<(ostream& o, diff_category c)): Support the new
	HARMLESS_UNION_CHANGE_CATEGORY.
	* tests/data/test-diff-filter/test-PR24731-report-0.txt: Likewise.
	* tests/data/test-diff-filter/test-PR24731-report-1.txt: Likewise.
	* tests/data/test-diff-filter/test-PR24731-v0.c: Likewise.
	* tests/data/test-diff-filter/test-PR24731-v0.o: Likewise.
	* tests/data/test-diff-filter/test-PR24731-v1.c: Likewise.
	* tests/data/test-diff-filter/test-PR24731-v1.o: Likewise.
	* tests/data/Makefile.am: Add the new test material above to
	source distribution.
	* tests/test-diff-filter.cc (in_out_spec): Add the new test input
	to this test harness.

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