RFR: Merge lworld [v2]

[Vicente Romero]

> Merge branch 'lworld' into bworld_merge_lworld
> # Conflicts:
> #	test/langtools/tools/javac/diags/examples/ReturnBeforeSuperclassInit.java

Vicente Romero has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 116 commits:

 - Merge branch 'lworld' into bworld_merge_lworld
   
   # Conflicts:
   #	test/langtools/tools/javac/diags/examples/ReturnBeforeSuperclassInit.java
 - Classfile signature bang support
   
   Reviewed-by: mcimadamore
 - add different flavors to the useSiteNullChecks compiler option
   
   Reviewed-by: mcimadamore
 - Minor cleanup, preview and since
   
   Reviewed-by: vromero
 - add use site null checks for fields
   
   Reviewed-by: liach, mcimadamore
 - Support for null-restricted types in JShell
   
   Co-authored-by: Maurizio Cimadamore <mcimadamore at openjdk.org>
   Reviewed-by: mcimadamore
 - Fixing null restricted types for fields/methods in compact source files.
   
   Reviewed-by: mcimadamore
 - add use site null checks
   
   Reviewed-by: mcimadamore
 - add the NullRestricted annotation to null restricted fields of value classes
   
   Reviewed-by: mcimadamore
 - Disable null-restricted varargs
   
   Reviewed-by: vromero
 - ... and 106 more: https://git.openjdk.org/valhalla/compare/43b6a738...a3783e4a

-------------

Changes: https://git.openjdk.org/valhalla/pull/1962/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1962&range=01
  Stats: 7461 lines in 118 files changed: 7195 ins; 57 del; 209 mod
  Patch: https://git.openjdk.org/valhalla/pull/1962.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1962/head:pull/1962

PR: https://git.openjdk.org/valhalla/pull/1962