RFR: 7903674: jextract should ignore non-enum constants inside enum [v4]

[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.

Athijegannathan Sundararajan has updated the pull request incrementally with one additional commit since the last revision:

  reduce the diffs for this patch.

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Changes:
  - all: https://git.openjdk.org/jextract/pull/221/files
  - new: https://git.openjdk.org/jextract/pull/221/files/2ceb8490..aca001e3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=221&range=03
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=221&range=02-03

  Stats: 3 lines in 1 file changed: 0 ins; 1 del; 2 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