RFR: 7903674: jextract should ignore non-enum constants inside enum

[Athijegannathan Sundararajan]

* checking cursor kind of child cursor to be CursorKind.EnumConstantDecl. 
* Piggybacking to fix similar check of record child element kinds to be CursorKind.FieldDecl.  
* Added null check for createTree return value in collectNestedTypes method as well.
* In addition to jextract tests, I ran all samples on Mac OS. All fine with this change. Also, ran jextract script for Mac OS from  https://github.com/manuelbl/JavaDoesUSB.git.

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Commit messages:
 - 7903674: jextract should ignore non-enum constants inside enum

Changes: https://git.openjdk.org/jextract/pull/221/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=221&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903674
  Stats: 18 lines in 1 file changed: 7 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jextract/pull/221.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/221/head:pull/221

PR: https://git.openjdk.org/jextract/pull/221