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

[Coleen Phillimore]

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.

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Commit messages:
 - 8309599: WeakHandle and OopHandle release should clear obj pointer

Changes: https://git.openjdk.org/jdk/pull/15920/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15920&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309599
  Stats: 13 lines in 8 files changed: 2 ins; 1 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/15920.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15920/head:pull/15920

PR: https://git.openjdk.org/jdk/pull/15920