go.tools/cmd/vet: use new Assertable predicate (fix build)

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/59210045

cmd/vet/types.go

@@ -103,7 +103,7 @@ func (f *File) matchArgTypeInternal(t printfArgType, typ types.Type, arg ast.Exp
 	}
 	// If we can use a string, might arg (dynamically) implement the Stringer or Error interface?
 	if t&argString != 0 {
-		if types.Implements(typ, errorType, false) || types.Implements(typ, stringerType, false) {
+		if types.Assertable(errorType, typ) || types.Assertable(stringerType, typ) {
 			return true
 		}
 	}