Reduce exposure of FL_FREEZE

The `FL_FREEZE` flag is redundant with `SHAPE_ID_FL_FROZEN`, so
ideally it should be eliminated in favor of the later.

Doing so would eliminate the risk of desync between the two, but
also solve the problem of the frozen status being global in namespace
context (See Bug #21330).

class.c

@@ -2771,7 +2771,8 @@ rb_freeze_singleton_class(VALUE x)
     if (!RCLASS_SINGLETON_P(x)) {
         VALUE klass = RBASIC_CLASS(x);
         if (klass && // no class when hidden from ObjectSpace
-            FL_TEST(klass, (FL_SINGLETON|FL_FREEZE)) == FL_SINGLETON) {
+            FL_TEST_RAW(klass, FL_SINGLETON) &&
+            !OBJ_FROZEN_RAW(klass)) {
             OBJ_FREEZE(klass);
         }
     }