[code-reflection] RFR: Block.Reference implements CodeItem

[Paul Sandoz]

`Block.Reference` implements `CodeItem`, thereby all items in a code model share the same top type.

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Commit messages:
 - Block.Reference implements CodeItem.

Changes: https://git.openjdk.org/babylon/pull/478/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=478&range=00
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/babylon/pull/478.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/478/head:pull/478

PR: https://git.openjdk.org/babylon/pull/478