RFR: More parser fixes for null restricted types

[Maurizio Cimadamore]

This PR fixes a number of leftover issues in the parser:
* sometimes the parser accepts `!` before the type arguments of a generic type
* null restrictions parsed as part of the array type in an array creation expression are dropped on the floor
* null restrictions appearing (erroneously) before an array initializer causes spurious parser errors, as parsers doesn't take bangs into account

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/valhalla/pull/1868/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1868&range=00
  Stats: 58 lines in 4 files changed: 38 ins; 4 del; 16 mod
  Patch: https://git.openjdk.org/valhalla/pull/1868.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1868/head:pull/1868

PR: https://git.openjdk.org/valhalla/pull/1868