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

[Vicente Romero]

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

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Commit messages:
 - 8338910: [lw5] enhance the Signature attribute to represent nullability

Changes: https://git.openjdk.org/valhalla/pull/1223/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1223&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338910
  Stats: 479 lines in 5 files changed: 470 ins; 1 del; 8 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