RFR: JDK-8068925: Add @Override in javax.tools classes

[Jonathan Gibbons]

Please review a simple update to add `@Override` annotations where appropriate in the `javax.tools` API, and one other minor opportunistic cleanup to use an `instanceof` pattern.

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Commit messages:
 - JDK-8068925: Add @Override in javax.tools classes

Changes: https://git.openjdk.org/jdk/pull/13766/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13766&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8068925
  Stats: 21 lines in 4 files changed: 17 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13766.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13766/head:pull/13766

PR: https://git.openjdk.org/jdk/pull/13766