c++: Remove cdtor_label

c++: Remove cdtor_label

Jakub pointed out that cdtor_label is unnecessary, we should get all the
desired semantics with a normal return.

gcc/cp/ChangeLog:

	* cp-tree.h (struct language_function): Remove x_cdtor_label.
	(cdtor_label, LABEL_DECL_CDTOR): Remove.
	* constexpr.cc (returns): Don't check LABEL_DECL_CDTOR.
	(cxx_eval_constant_expression): Don't call returns.
	* decl.cc (check_goto): Don't check cdtor_label.
	(start_preparsed_function): And don't set it.
	(finish_constructor_body, finish_destructor_body): Remove.
	(finish_function_body): Don't call them.
	* typeck.cc (check_return_expr): Handle cdtor_returns_this here.
	* semantics.cc (finish_return_stmt): Not here.