If we delete `declare_toplevel_type(HeapWord* volatile)` in `src/hotspot/share/gc/shared/vmStructs_gc.hpp`, `make CONF=debug images` will fail with sth like:

```
assert(findType("HeapWord* volatile") != 0) failed: type "HeapWord* volatile" not found in type table
```

The failure is due to `recursiveFindType` doesn't strip `volatile` type qualifier. (It supports `const` in contrast.)