RFR: More parser fixes for null restricted types [v2]

[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

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

  Add better parsing logic for method references

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1868/files
  - new: https://git.openjdk.org/valhalla/pull/1868/files/05d64fe6..f56d1193

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1868&range=01
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1868&range=00-01

  Stats: 49 lines in 3 files changed: 33 ins; 9 del; 7 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