Optimization: do not generate oneofs for proto3 optional fields.

Proto3 optional fields should use a hasbit for their presence.  But we had been giving them oneof layouts, which makes them unnecessarily large.  This CL will shrink messages with proto3 optional fields, by using hasbits instead of oneof cases for them.

PiperOrigin-RevId: 485998527

upb/reflection/message_def.c

@@ -496,7 +496,7 @@ static bool _upb_MessageDef_EncodeMessage(upb_DescState* s,
     s->ptr = upb_MtDataEncoder_PutField(&s->e, s->ptr, type, number, modifiers);
   }
 
-  for (int i = 0; i < m->oneof_count; i++) {
+  for (int i = 0; i < m->real_oneof_count; i++) {
     if (!_upb_DescState_Grow(s, a)) return false;
     s->ptr = upb_MtDataEncoder_StartOneof(&s->e, s->ptr);