RFR: 8338910: [lw5] enhance the Signature attribute to represent nullability [v2]

[Vicente Romero]

> Enhance the Signature attribute so that we can represent nullability and null restrictions

Vicente Romero has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - merge with lw5
 - 8338910: [lw5] enhance the Signature attribute to represent nullability

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Changes: https://git.openjdk.org/valhalla/pull/1223/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1223&range=01
  Stats: 477 lines in 5 files changed: 471 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/valhalla/pull/1223.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1223/head:pull/1223

PR: https://git.openjdk.org/valhalla/pull/1223