RFR: 8071507: (ref) Clear phantom reference as soft and weak references do

[Kim Barrett]

Please review this change to PhantomReference processing, changing the
GC-based notification to automatically clear the referent.

This change provides performance benefits by eliminating the work
involved in keeping the otherwise inaccessible referent objects alive,
as required by the existing specification. This not only immediately
removes some work, but may enable further performance improvements.
It also allows the referent objects to be immediately reclaimed in
the GC cycle in which they were determined to be inaccessible, rather
than lingering as a form of floating garbage until the application
deals with the notified reference.

This change results in a behavioral change to application code, as
demonstrated by the associated test.  Under the old specification, a
reference R with referent X may be kept alive because it is referenced
by an otherwise inaccessible referent Y of phantom reference P.  This
will result in X being treated as strongly referenced and prevent R
from being notified, even if R is a phantom reference and X has become
inaccessible to the application.  With this change, Y is reclaimed
when it becomes inaccessible and P is notified, and no longer prevents
X from itself becoming a candidate for reclaimation once it is no
longer accessible to the application.  While this is a change in
behavior, it seems unlikely to affect applications negatively.

CR:
https://bugs.openjdk.java.net/browse/JDK-8071507

Webrevs:
http://cr.openjdk.java.net/~kbarrett/8071507/jdk.05/
http://cr.openjdk.java.net/~kbarrett/8071507/hotspot.05/

Testing:
jprt, aurora ad hoc (defaults, GC/Runtime nightly, JCK)