RFR: 8350905: Releasing a WeakHandle's referent may extend its lifetime

[William Kemper]

When weak handles are cleared, the `nullptr` is stored with the `ON_PHANTOM_OOP_REF` decorator. For concurrent collectors using a SATB barrier, this may cause the referent to be enqueued and marked when it would be otherwise unreachable. The problem is especially acute for Shenandoah's generational mode, in which a young region holding the otherwise unreachable referent, may become trash after the referent is enqueued for old marking. We are proposing that native weak references are cleared with an additional `AS_NO_KEEPALIVE` decorator. This is similar to what was done for j.l.r.WeakReference in [JDK-8240696](https://bugs.openjdk.org/browse/JDK-8240696).

# Testing

GHA, `hotspot_gc_shenandoah`. Additionally, for G1, ZGC, and Shenandoah we've run Extremem, Dacapo, SpecJVM2008, SpecJBB2015, Heapothesys and Diluvian. All executions completed without errors.

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Commit messages:
 - Also, no keep alive when releasing string dedup reference
 - Use AS_NO_KEEPALIVE when clearing weak referent

Changes: https://git.openjdk.org/jdk/pull/23935/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23935&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350905
  Stats: 3 lines in 3 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/23935.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23935/head:pull/23935

PR: https://git.openjdk.org/jdk/pull/23935