RFR: 7903822: Add toString for some internal types

[Jonathan Gibbons]

Please review a trivial update to add a useful definition of `.toString()` for some internal classes.

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Commit messages:
 - CODETOOLS-7903822: Add toString for some internal types

Changes: https://git.openjdk.org/apidiff/pull/5/files
  Webrev: https://webrevs.openjdk.org/?repo=apidiff&pr=5&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903822
  Stats: 13 lines in 2 files changed: 13 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/apidiff/pull/5.diff
  Fetch: git fetch https://git.openjdk.org/apidiff.git pull/5/head:pull/5

PR: https://git.openjdk.org/apidiff/pull/5