RFR: 8374027: Restrict parser to only accept null-restrictions in selected locations

[Maurizio Cimadamore]

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Commit messages:
 - Clean up code and add comments
 - Revert tests
 - Initial push

Changes: https://git.openjdk.org/valhalla/pull/1830/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1830&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374027
  Stats: 141 lines in 7 files changed: 58 ins; 49 del; 34 mod
  Patch: https://git.openjdk.org/valhalla/pull/1830.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1830/head:pull/1830

PR: https://git.openjdk.org/valhalla/pull/1830