[code-reflection] RFR: Define JavaRef top-type for all Java refs [v2]

[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.

Paul Sandoz has updated the pull request incrementally with one additional commit since the last revision:

  Revert unintended changes.

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Changes:
  - all: https://git.openjdk.org/babylon/pull/411/files
  - new: https://git.openjdk.org/babylon/pull/411/files/e3a74b3f..408353c6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=babylon&pr=411&range=01
 - incr: https://webrevs.openjdk.org/?repo=babylon&pr=411&range=00-01

  Stats: 26 lines in 2 files changed: 9 ins; 3 del; 14 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