`PSScavengeCLDClosure` is a closure applied on CLD to scavenge cld-to-young pointers. One can query `has_modified_oops` to check if a CLD potentially has any such pointers. Every cld-to-young pointer requires copying the corresponding object to to-space/old-gen. After the copying, depending on the destination, we need to call `record_modified_oops` to re-establish the variant that having cld-to-young pointers implies `has_modified_oops == true`.

Currently, the logic is recording the being-processed CLD in the oop-closure and let oop-closure to invoke `record_modified_oops` if needed. Alternatively, we can let oop-closure collect if-has-to-young pointers and the CLD-closure deal with any CLD-specific logic. This way, all CLD related logic is encapsulated inside the CLD-closure.