Some calls involving `ClassUnloadingContext` and `ClassLoaderDataGraph` are issued by the caller of `SerialFullGC::invoke_at_safepoint`, but most class unloading work is performed inside `invoke_at_safepoint`. Try to group those related logic together.

Note: Serial full-gc still accesses klasses of dead-objs after phase1 marking, because there is no bitmap for liveness of objs. Therefore, it's invalid to do group all classunloading logic like what is done in JDK-8361404.