RFR: JDK-8307563: make most fields final in `JavacTrees`

[Jonathan Gibbons]

Please review a small cleanup fix to address a comment left a long time back.

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Commit messages:
 - JDK-8307563: make most fields final in `JavacTrees`

Changes: https://git.openjdk.org/jdk/pull/13847/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13847&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307563
  Stats: 43 lines in 1 file changed: 16 ins; 25 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13847.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13847/head:pull/13847

PR: https://git.openjdk.org/jdk/pull/13847