RFR: 7903715: Jextract generates duplicate allocator parameters

[Maurizio Cimadamore]

This simple fix addresses a case where jextract generates two function parameter with duplicate name `allocator`.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/jextract/pull/238/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=238&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903715
  Stats: 75 lines in 3 files changed: 71 ins; 4 del; 0 mod
  Patch: https://git.openjdk.org/jextract/pull/238.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/238/head:pull/238

PR: https://git.openjdk.org/jextract/pull/238