* gc.c: rename is_dead_object() to is_dying_object().
  This function is not opposite against is_live_object()
  because is_dying_object() does *not* check object type.
* gc.c (is_dying_object): change condition.
* gc.c (is_live_object): use T_NONE instead of 0.
* gc.c (rb_objspace_dying_object_p): added.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46716 b2dd03c8-39d4-4d8f-98ff-823fe69b080e