Move class pre-aging out of the allocation path

Previously classes and modules were pre-aged. Ie. as soon as they're
allocated they are aged to old_age - 1. This was done with the
assumption that classes are generally always long lived, so we should
assume that any that survive a single GC can be considered old.

This commit keeps the same semantics, but moves the logic out of the
allocation path, in order to simplify allocation. Classes and modules
are now set to old-age the first time they are marked.

Author: eightbitraptor

gc/default/default.c

@@ -2175,10 +2175,6 @@ newobj_init(VALUE klass, VALUE flags, int wb_protected, rb_objspace_t *objspace,
     RBASIC(obj)->shape_id = 0;
 #endif
 
-    int t = flags & RUBY_T_MASK;
-    if (t == T_CLASS || t == T_MODULE || t == T_ICLASS) {
-        RVALUE_AGE_SET_CANDIDATE(objspace, obj);
-    }
 
 #if RACTOR_CHECK_MODE
     void rb_ractor_setup_belonging(VALUE obj);
@@ -4407,8 +4403,16 @@ gc_aging(rb_objspace_t *objspace, VALUE obj)
 
     if (!RVALUE_PAGE_WB_UNPROTECTED(page, obj)) {
         if (!RVALUE_OLD_P(objspace, obj)) {
-            gc_report(3, objspace, "gc_aging: YOUNG: %s\n", rb_obj_info(obj));
-            RVALUE_AGE_INC(objspace, obj);
+            int t = BUILTIN_TYPE(obj);
+            if (t == T_CLASS || t == T_MODULE || t == T_ICLASS) {
+                gc_report(3, objspace, "gc_aging: YOUNG class: %s\n", rb_obj_info(obj));
+                RVALUE_AGE_SET(obj, RVALUE_OLD_AGE);
+                RVALUE_OLD_UNCOLLECTIBLE_SET(objspace, obj);
+            }
+            else {
+                gc_report(3, objspace, "gc_aging: YOUNG: %s\n", rb_obj_info(obj));
+                RVALUE_AGE_INC(objspace, obj);
+            }
         }
         else if (is_full_marking(objspace)) {
             GC_ASSERT(RVALUE_PAGE_UNCOLLECTIBLE(page, obj) == FALSE);