re PR c++/772 (Statement expressions issues)

	PR c/772
	PR c/17913
	* c-tree.h (C_DECL_UNJUMPABLE_STMT_EXPR,
	C_DECL_UNDEFINABLE_STMT_EXPR, struct c_label_list, struct
	c_label_context, label_context_stack): New.
	* c-decl.c (define_label): Check for jumps into statement
	expressions.  Add label to list of defined labels.
	(start_function): Push context on label_context_stack.
	(finish_function): Pop context from label_context_stack.
	* c-typeck.c (label_context_stack): New.
	(c_finish_goto_label): Check for jumps into statement
	expressions.  Add label to list of jumped to labels.
	(struct c_switch): Add blocked_stmt_expr.
	(c_start_case): Initialize it.
	(do_case): Check it.
	(c_finish_case): Verify !blocked_stmt_expr.
	(c_begin_stmt_expr):  Push context on label_context_stack.
	Increment blocked_stmt_expr.  Mark labels jumped to from outside
	as undefinable.
	(c_finish_stmt_expr): December blocked_stmt_expr.  Mark labels
	defined in the statement expression and no longer jumpable to.
	Mark labels jumped to from just outside the statement expression
	as again definable.  Pop context from label_context_stack.
	* doc/extend.texi (Statement Exprs): Update.

objc:
	* objc-act.c (objc_start_function): Push context on
	label_context_stack.

testsuite:
	* gcc.dg/stmt-expr-label-1.c, gcc.dg/stmt-expr-label-2.c,
	gcc.dg/stmt-expr-label-3.c : New tests.
	* gcc.c-torture/execute/medce-2.c: Remove.

From-SVN: r97273