[lworld] RFR: 8279368: [lworld] Add parser support for declaration of value classes

[Srikanth Adayapalam]

Enhance the parser to accept value class declarations

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Commit messages:
 - 8279368: Add parser support for declaration of value classes

Changes: https://git.openjdk.java.net/valhalla/pull/596/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=596&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279368
  Stats: 293 lines in 23 files changed: 239 ins; 9 del; 45 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/596.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/596/head:pull/596

PR: https://git.openjdk.java.net/valhalla/pull/596