[lworld] RFR: 8279840 [lworld] Inconsistent treatment of repeated modifiers

[Jesper Steen Møller]

Make context-dependent keywords value and primitive act more like ordinary modifiers (for consistent error reporting.)

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Commit messages:
 - 8279840 [lworld] Inconsistent treatment of repeated modifiers

Changes: https://git.openjdk.java.net/valhalla/pull/625/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=625&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279840
  Stats: 33 lines in 3 files changed: 29 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/625.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/625/head:pull/625

PR: https://git.openjdk.java.net/valhalla/pull/625