[code-reflection] RFR: Method code model uniquness

[Mourad Abbay]

Multiple `Method` objects that represent the same java method, have different code models. This is because every time we call `Class.getMethod` or `Class.getDeclaredMethod` we get a newly created instance. One possible solution is to attach the model to the root `Method` object.

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Commit messages:
 - Write test that shows the issue

Changes: https://git.openjdk.org/babylon/pull/415/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=415&range=00
  Stats: 32 lines in 1 file changed: 32 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/babylon/pull/415.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/415/head:pull/415

PR: https://git.openjdk.org/babylon/pull/415