RFR: 8203035: Implement equals() and hashCode() for Throwable

[Victor Toni]

Being able to compare instances of Throwable allows simple detection of exceptions raised by the same circumstances. Comparison allows for reduction of excessive logging e.g. in hotspots without requiring custom code to compare Throwables.

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Commit messages:
 - 8203035: Implement equals() and hashCode() for Throwable

Changes: https://git.openjdk.org/jdk/pull/11624/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11624&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8203035
  Stats: 329 lines in 2 files changed: 329 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11624.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11624/head:pull/11624

PR: https://git.openjdk.org/jdk/pull/11624