[C++ DR 2336] Clean up synth walkers first

[C++ DR 2336] Clean up synth walkers first

https://gcc.gnu.org/ml/gcc-patches/2018-11/msg01376.html
	* cp-tree.h (enum special_function_kind): Reorder and comment.
	* method.c (SFK_CTOR_P, SFK_DTOR_P, SFK_ASSIGN_P, SFK_COPY_P)
	(SFK_MOVE_P): New predicates.
	(walk_field_subobs, synthesized_method_base_walk): Drop
	copy_arg_p, move_p, assign_p args.  Use new SFK predicates.  Order
	parameters consistently.
	(synthesized_method_walk): Drop ctor_p, copy_arg_p, move_p,
	assign_p calculations.  Use new SFK predicates.  Adjust calls to
	worker functions.

From-SVN: r266180