Avoid calling a trivial default constructor.

Avoid calling a trivial default constructor.

	* class.c (default_ctor_p): New.
	(in_class_defaulted_default_constructor): Use it.
	(type_has_non_user_provided_default_constructor): Use it.
	* call.c (build_over_call): Handle trivial default constructor.
	* cp-tree.h: Declare default_ctor_p.

From-SVN: r239783