[C++ DR 2336] virtual dtors, exception specs & abstract classes

[C++ DR 2336] virtual dtors, exception specs & abstract classes

https://gcc.gnu.org/ml/gcc-patches/2018-11/msg01389.html
	DR 2336
	* cp-tree.h (enum special_function_kind): Add sfk_virtual_destructor.
	* method.c (type_has_trivial_fn): Add it.
	(SFK_DTOR_P): Likewise.
	(synthesized_method_base_walk): Don't check access of vbases of
	abstract classes when sfk_virtual_destructor.
	(synthesized_method_walk): Skip vbases of abstract classes except
	when sfk_virtual_destructor.
	(get_defaulted_eh_spec): Set sfk_virtual_destructor as needed.

	* g++.dg/cpp1y/pr79393-3.C: New.

From-SVN: r266188