Fix ASAN and don't check SPECIAL_CONST_P

Heap allocated objects are never special constants.  Since we're walking
the heap, we know none of these objects can be special.  Also, adding
the object to the freelist will poison the object, so we can't check
that the type is T_NONE after poison.

gc.c

@@ -8727,33 +8727,30 @@ gc_ref_update(void *vstart, void *vend, size_t stride, void * data)
 
     /* For each object on the page */
     for (; v != (VALUE)vend; v += stride) {
-        if (!SPECIAL_CONST_P(v)) {
-            void *poisoned = asan_poisoned_object_p(v);
-            asan_unpoison_object(v, false);
+        void *poisoned = asan_poisoned_object_p(v);
+        asan_unpoison_object(v, false);
 
-            switch (BUILTIN_TYPE(v)) {
-              case T_NONE:
-                heap_page_add_freeobj(objspace, page, v);
-                free_slots++;
-                break;
-              case T_MOVED:
-                break;
-              case T_ZOMBIE:
-                break;
-              default:
-                if (RVALUE_WB_UNPROTECTED(v)) {
-                    page->flags.has_uncollectible_shady_objects = TRUE;
-                }
-                if (RVALUE_PAGE_MARKING(page, v)) {
-                    page->flags.has_remembered_objects = TRUE;
-                }
-                gc_update_object_references(objspace, v);
+        switch (BUILTIN_TYPE(v)) {
+          case T_NONE:
+            heap_page_add_freeobj(objspace, page, v);
+            free_slots++;
+            break;
+          case T_MOVED:
+            break;
+          case T_ZOMBIE:
+            break;
+          default:
+            if (RVALUE_WB_UNPROTECTED(v)) {
+                page->flags.has_uncollectible_shady_objects = TRUE;
             }
+            if (RVALUE_PAGE_MARKING(page, v)) {
+                page->flags.has_remembered_objects = TRUE;
+            }
+            gc_update_object_references(objspace, v);
+        }
 
-            if (poisoned) {
-                GC_ASSERT(BUILTIN_TYPE(v) == T_NONE);
-                asan_poison_object(v);
-            }
+        if (poisoned) {
+            asan_poison_object(v);
         }
     }