re PR c++/57319 (bogus "defaulted move assignment for ... calls a non-trivial move assignment operator for virtual base ...")

	PR c++/57319
	* class.c (vbase_has_user_provided_move_assign): New.
	* method.c (synthesized_method_walk): Check it.
	* cp-tree.h: Declare it.

From-SVN: r199114