* method.c (synthesized_method_walk): Track constexprness too.
	(process_subob_fn, walk_field_subobs): Likewise.
	(implicitly_declare_fn): Set DECL_DECLARED_CONSTEXPR_P.
	(defaulted_late_check): Handle DECL_DECLARED_CONSTEXPR_P.
	* class.c (add_implicitly_declared_members): Handle
	constexpr default ctor.

Co-Authored-By: Jason Merrill <jason@redhat.com>

From-SVN: r166014