Fix lambdas in template default argument of inherited ctor.

	* method.c (synthesized_method_base_walk): Replace an inherited
	template with its specialization.
	(synthesized_method_walk): Make inheriting_ctor a pointer.
	(maybe_explain_implicit_delete, explain_implicit_non_constexpr)
	(deduce_inheriting_ctor, implicitly_declare_fn): Adjust.

From-SVN: r251426