[code-reflection] RFR: Build extern type element tree

[Paul Sandoz]

The model that builds the model now explicitly builds the externalized type element tree rather than parsing it from the string form. Externalized type element instances are reused.

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Commit messages:
 - Remove unused field.
 - Build extern type element tree

Changes: https://git.openjdk.org/babylon/pull/419/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=419&range=00
  Stats: 86 lines in 3 files changed: 77 ins; 4 del; 5 mod
  Patch: https://git.openjdk.org/babylon/pull/419.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/419/head:pull/419

PR: https://git.openjdk.org/babylon/pull/419