RFR: 8309599: WeakHandle and OopHandle release should clear obj pointer

[Kim Barrett]

On Tue, 26 Sep 2023 12:47:42 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

> This change makes WeakHandle and OopHandle release null out the obj pointer, at the cost of making the release function non-const and some changes that propagated from that.  This enables ObjectMonitor code to test for null to see if the obj was already released, and seems like the right thing to do.  See comments from related PR in the bug report.
> Tested with tier1-4.

Looks good.

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Marked as reviewed by kbarrett (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/15920#pullrequestreview-1647918875