Categorize CV qualifier changes on fn return types as harmless

Categorize CV qualifier changes on fn return types as harmless

This partially fixes PR23700.

A change in the CV qualifiers of a function return value type should
be categorized as harmless.  And this is what this patch does.

	* include/abg-comparison.h (FN_RETURN_TYPE_CV_CHANGE_CATEGORY):
	New enumerator for diff_category.
	(EVERYTHING_CATEGORY): Update.
	* src/abg-comp-filter.cc (type_diff_has_cv_qual_change_only):
	Factorize this function out of ...
	(has_fn_parm_type_cv_qual_change): ... this one.
	(has_fn_return_type_cv_qual_change): Define new static function.
	(categorize_harmless_diff_node): Use the new
	has_fn_return_type_cv_qual_change.
	* src/abg-comparison.cc (get_default_harmless_categories_bitmap):
	Adjust to add the new FN_RETURN_TYPE_CV_CHANGE_CATEGORY category.
	(operator<<(ostream& o, diff_category c)): Support the new
	FN_RETURN_TYPE_CV_CHANGE_CATEGORY.
	* tests/data/Makefile.am: Add the new test material below to
	source distribution.
	* tests/data/test-diff-filter/test46-fn-return-qual-change-report-0.txt:
	New reference output for the new input test.
	* tests/data/test-diff-filter/test46-fn-return-qual-change-v{0,1}.c:
	New source code for the new binary test input.
	* tests/data/test-diff-filter/test46-fn-return-qual-change-v{0,1}.o:
	New binary test input files.
	* tests/test-diff-filter.cc: Add the new test input above to test
	harness.

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