RFR: 8338874: [lw5] add nullable types [v4]

[Vicente Romero]

> adding support for nullable types

Vicente Romero has updated the pull request incrementally with one additional commit since the last revision:

  add a flag for null restrictions for value classes only

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1219/files
  - new: https://git.openjdk.org/valhalla/pull/1219/files/eb0e2c00..72086bb5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1219&range=03
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1219&range=02-03

  Stats: 48 lines in 3 files changed: 38 ins; 2 del; 8 mod
  Patch: https://git.openjdk.org/valhalla/pull/1219.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1219/head:pull/1219

PR: https://git.openjdk.org/valhalla/pull/1219