system.h: Forward-declare struct rtx_def, struct rtvec_def, and union tree_node.

system.h: Forward-declare struct rtx_def, struct rtvec_def, and union tree_node.

	* system.h: Forward-declare struct rtx_def, struct rtvec_def,
	and union tree_node.  Typedef rtx, rtvec, and tree.  Fix comment.
	* rtl.h: Don't forward-declare union tree_node here.  Don't
	typedef rtx or rtvec here.  Change all uses of struct rtx_def *,
	struct rtvec_def *, or union tree_node * to use rtx, rtvec,
	and tree respectively.
	* tree.h: Don't forward-declare struct rtx_def here.  Don't
	typedef tree here.  Change all uses of struct rtx_def *,
	struct rtvec_def *, or union tree_node * to use rtx, rtvec,
	and tree respectively.

From-SVN: r44980