RFR: 7903360: jextract does not handle function pointer return type properly

[Athijegannathan Sundararajan]

* generating functional interface for pointer to function return types
* taking care of javadoc comment by using nameAndType function uniformly

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Commit messages:
 - 7903360: jextract does not handle function pointer return type properly

Changes: https://git.openjdk.org/jextract/pull/90/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=90&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903360
  Stats: 73 lines in 5 files changed: 56 ins; 7 del; 10 mod
  Patch: https://git.openjdk.org/jextract/pull/90.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/90/head:pull/90

PR: https://git.openjdk.org/jextract/pull/90