[lworld] RFR: 8338252: [lworld] jdk/internal/ReferencedKeyTest fix merge and robustness

[Roger Riggs]

Fixup after merge.
Addressing latent issues with ReferencedKeyTest
- During the `methods()` tests the keys should be strongly held to avoid inadvertent GC collection and subsequent test failures (JDK-8336926)
- Merge changes from Valhalla to use String (identity objects) for keys instead of Integer and Long that are value objects.

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Commit messages:
 - add bugid
 - 8338252: [lworld] jdk/internal/ReferencedKeyTest fix merge and robustness

Changes: https://git.openjdk.org/valhalla/pull/1201/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1201&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338252
  Stats: 43 lines in 1 file changed: 22 ins; 4 del; 17 mod
  Patch: https://git.openjdk.org/valhalla/pull/1201.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1201/head:pull/1201

PR: https://git.openjdk.org/valhalla/pull/1201