AOT caching of WeakReference

[ioi.lam at oracle.com]

Here's my  experiment with storing weak references in the AOT cache

https://github.com/iklam/jdk/blob/8341587-example-of-weakref-problem-in-aot-cache/AOT-Cache-And-WeakRef.md

As discussed in the Leyden meeting today, there seem to be at least two 
issues

- In premain, CDS keeps every archived heap object alive (look for 
UsePermanentHeapObjects in the code). I plan to get rid of this shortly. 
UsePermanentHeapObjects is intended for the AOT compiler to access heap 
objects, but it should be implemented differently (only remember objects 
that are required by AOT code, which should include only Strings and 
class mirrors).

- WeakReference.queue is not pointing to the production-time value of 
ReferenceQueue.NULL


Thanks

- Ioi