RFR: Disable null-restricted varargs

[Maurizio Cimadamore]

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

-------------

Commit messages:
 - Initial push

Changes: https://git.openjdk.org/valhalla/pull/1913/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1913&range=00
  Stats: 13 lines in 4 files changed: 11 ins; 1 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