RFR: Disable null-restricted varargs [v2]

[Maurizio Cimadamore]

> It should not be possible to use a null-restricted varargs element type, the parser should reject that combination.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Add tests for varargs record component

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1913/files
  - new: https://git.openjdk.org/valhalla/pull/1913/files/657f3d58..25918b7b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1913&range=01
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1913&range=00-01

  Stats: 7 lines in 3 files changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/valhalla/pull/1913.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1913/head:pull/1913

PR: https://git.openjdk.org/valhalla/pull/1913