[code-reflection] RFR: Build ref attributes from externalized type elems

[Paul Sandoz]

This removes the dependence on specific parsing of refs. Now they are type elements we can use its string form.

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Commit messages:
 - Build ref attributes from externalized type elems

Changes: https://git.openjdk.org/babylon/pull/418/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=418&range=00
  Stats: 28 lines in 1 file changed: 0 ins; 28 del; 0 mod
  Patch: https://git.openjdk.org/babylon/pull/418.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/418/head:pull/418

PR: https://git.openjdk.org/babylon/pull/418