object.c: improve fake_class_p to also handle T_MODULE

This requires ensuring T_MODULE never has FL_SINGLETON set,
so RMODULE_IS_REFINEMENT had to be moved.

class.c

@@ -62,8 +62,8 @@
  * 0:    RCLASS_IS_ROOT
  *           The class has been added to the VM roots. Will always be marked and pinned.
  *           This is done for classes defined from C to allow storing them in global variables.
- * 1:    RMODULE_IS_REFINEMENT
- *           Module is used for refinements.
+ * 1:    <reserved>
+ *          Ensures that RUBY_FL_SINGLETON is never set on a T_MODULE. See `rb_class_real`.
  * 2:    RCLASS_PRIME_CLASSEXT_PRIME_WRITABLE
  *           This module's prime classext is the only classext and writable from any namespaces.
  *           If unset, the prime classext is writable only from the root namespace.
@@ -71,6 +71,8 @@
  *           Module has been initialized.
  * 4:    RCLASS_NAMESPACEABLE
  *           Is a builtin class that may be namespaced. It larger than a normal class.
+ * 5:    RMODULE_IS_REFINEMENT
+ *           Module is used for refinements.
  */
 
 #define METACLASS_OF(k) RBASIC(k)->klass

object.c

@@ -278,25 +278,25 @@ rb_obj_not_equal(VALUE obj1, VALUE obj2)
     return rb_obj_not(result);
 }
 
-VALUE
-rb_class_real(VALUE cl)
-{
-    while (cl &&
-        (RCLASS_SINGLETON_P(cl) || BUILTIN_TYPE(cl) == T_ICLASS)) {
-        cl = RCLASS_SUPER(cl);
-    }
-    return cl;
-}
-
 static inline VALUE
 fake_class_p(VALUE klass)
 {
-    RUBY_ASSERT(RB_TYPE_P(klass, T_CLASS) || RB_TYPE_P(klass, T_ICLASS));
-    STATIC_ASSERT(t_iclass_overlap, !(T_CLASS & T_ICLASS));
+    RUBY_ASSERT(RB_TYPE_P(klass, T_CLASS) || RB_TYPE_P(klass, T_MODULE) || RB_TYPE_P(klass, T_ICLASS));
+    STATIC_ASSERT(t_iclass_overlap_t_class, !(T_CLASS & T_ICLASS));
+    STATIC_ASSERT(t_iclass_overlap_t_module, !(T_MODULE & T_ICLASS));
 
     return FL_TEST_RAW(klass, T_ICLASS | FL_SINGLETON);
 }
 
+VALUE
+rb_class_real(VALUE cl)
+{
+    while (RB_UNLIKELY(cl && fake_class_p(cl))) {
+        cl = RCLASS_SUPER(cl);
+    }
+    return cl;
+}
+
 VALUE
 rb_obj_class(VALUE obj)
 {