[code-reflection] Integrated: Use SequencedSet

[Paul Sandoz]

Use `SequencedSet`, ensuring easier access to a first element.

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Commit messages:
 - Use SequencedSet

Changes: https://git.openjdk.org/babylon/pull/834/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=834&range=00
  Stats: 26 lines in 3 files changed: 3 ins; 5 del; 18 mod
  Patch: https://git.openjdk.org/babylon/pull/834.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/834/head:pull/834

PR: https://git.openjdk.org/babylon/pull/834