RFR: 7903630: jextract emits duplicate symbols for macro clashing with enum constants

[Maurizio Cimadamore]

The removal of `EnumConstantLifter` caused certain name conflicts to go undetected by `DuplicateFilter`. More specifically, if a macro constant has same name as that of an enum constant, given the enum constant is no longer lifted to toplevel, `DuplicateFilter` will not see the clash. The fix is to tweak `DuplicateFilter` to recurse into enums.

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Commit messages:
 - Add code comment
 - Initial push

Changes: https://git.openjdk.org/jextract/pull/177/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=177&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903630
  Stats: 15 lines in 3 files changed: 15 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jextract/pull/177.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/177/head:pull/177

PR: https://git.openjdk.org/jextract/pull/177