[MetadataReader] Adapt to newer Objective-C class_rw_t
Newer Objective-C runtimes implement a size optimization in class_rw_t
which requires an additional indirection to get to the class_ro_t pointer.
Thanks to Davide for getting to the bottom of this!
(cherry picked from commit 20573bdbb1404052b50ec3940a2682cf7d55e1f9)