[code-reflection] RFR: Define JavaRef top-type for all Java refs

[Paul Sandoz]

This represents a modest step towards bringing java refs into the code model as type elements, and perhaps more formally enabling traversal and transformation of a type element's tree structure more directly than doing so via externalization.

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Commit messages:
 - Define JavaRef top-type for all Java refs.

Changes: https://git.openjdk.org/babylon/pull/411/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=411&range=00
  Stats: 62 lines in 7 files changed: 35 ins; 9 del; 18 mod
  Patch: https://git.openjdk.org/babylon/pull/411.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/411/head:pull/411

PR: https://git.openjdk.org/babylon/pull/411