Fix ractor move of object with generic ivars (ruby#15056)

This bug was happening only when the `id2ref` table exists. We need
to replace the generic fields before replacing the object id of the
newly moved object.

Fixes [Bug #21664]

Author: luke-gruber

bootstraptest/test_ractor.rb

@@ -1969,6 +1969,19 @@ def ==(o)
   roundtripped_obj.instance_variable_get(:@array1) == [1] ? :ok : roundtripped_obj
 }
 
+# move object with generic ivars and existing id2ref table
+# [Bug #21664]
+assert_equal 'ok', %q{
+  obj = [1]
+  obj.instance_variable_set("@field", :ok)
+  ObjectSpace._id2ref(obj.object_id) # build id2ref table
+
+  ractor = Ractor.new { Ractor.receive }
+  ractor.send(obj, move: true)
+  obj = ractor.value
+  obj.instance_variable_get("@field")
+}
+
 # copy object with complex generic ivars
 assert_equal 'ok', %q{
   # Make Array too_complex

ractor.c

@@ -1973,13 +1973,12 @@ move_leave(VALUE obj, struct obj_traverse_replace_data *data)
     rb_gc_writebarrier_remember(data->replacement);
 
     void rb_replace_generic_ivar(VALUE clone, VALUE obj); // variable.c
-
-    rb_gc_obj_id_moved(data->replacement);
-
     if (UNLIKELY(rb_obj_exivar_p(obj))) {
         rb_replace_generic_ivar(data->replacement, obj);
     }
 
+    rb_gc_obj_id_moved(data->replacement);
+
     VALUE flags = T_OBJECT | FL_FREEZE | (RBASIC(obj)->flags & FL_PROMOTED);
 
     // Avoid mutations using bind_call, etc.