c++: -Wdangling-reference and empty class [PR115361]

c++: -Wdangling-reference and empty class [PR115361]

We can't have a dangling reference to an empty class unless it's
specifically to that class or one of its bases.  This was giving a
false positive on the _ExtractKey pattern in libstdc++ hashtable.h.

This also adjusts the order of arguments to reference_related_p, which
is relevant for empty classes (unlike scalars).

Several of the classes in the testsuite needed to gain data members to
continue to warn.

	PR c++/115361

gcc/cp/ChangeLog:

	* call.cc (do_warn_dangling_reference): Check is_empty_class.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/attr-no-dangling6.C
	* g++.dg/ext/attr-no-dangling7.C
	* g++.dg/ext/attr-no-dangling8.C
	* g++.dg/ext/attr-no-dangling9.C
	* g++.dg/warn/Wdangling-reference1.C
	* g++.dg/warn/Wdangling-reference2.C
	* g++.dg/warn/Wdangling-reference3.C: Make classes non-empty.
	* g++.dg/warn/Wdangling-reference23.C: New test.