* constexpr.c (cxx_eval_loop_expr, cxx_eval_switch_expr): New.
	(cxx_eval_statement_list): New.
	(cxx_eval_constant_expression): Handle LABEL_EXPR,
	CASE_LABEL_EXPR, GOTO_EXPR, LOOP_EXPR, SWITCH_EXPR.  Handle jump
	semantics of RETURN_EXPR.
	(many functions): Add jump_target parameter.
	(returns, breaks, continues, switches, label_matches): New.
	* cp-tree.h (LABEL_DECL_BREAK, LABEL_DECL_CONTINUE): New.
	* cp-gimplify.c (begin_bc_block): Set them.

From-SVN: r217670