RFR: 7903674: jextract should ignore non-enum constants inside enum [v3]

[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:

  addressed review comments. added a test.

-------------

Changes:
  - all: https://git.openjdk.org/jextract/pull/221/files
  - new: https://git.openjdk.org/jextract/pull/221/files/ab4c28a1..2ceb8490

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=221&range=02
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=221&range=01-02

  Stats: 55 lines in 5 files changed: 44 ins; 3 del; 8 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