RFR: 8338910: [lw5] enhance the Signature attribute to represent nullability [v3]

[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 12 commits:

 - erasure was not removing nullness markers
 - changes to NullRestricted parsing
 - change to SignaturesImpl
 - addressing review comments
 - fixing bugs
 - adding test case
 - removing commented code
 - Merge with lw5
 - fixing bugs
 - array support, addressing review comments
 - ... and 2 more: https://git.openjdk.org/valhalla/compare/4999dfe1...07a53fe5

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Changes: https://git.openjdk.org/valhalla/pull/1223/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1223&range=02
  Stats: 623 lines in 7 files changed: 612 ins; 6 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