Integrated: Merge lworld

[Vicente Romero]

Merge branch 'lworld' into bworld_merge_lworld
# Conflicts:
#	src/jdk.compiler/share/classes/com/sun/tools/javac/jvm/Gen.java

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Commit messages:
 - Merge branch 'lworld' into bworld_merge_lworld
 - improvements to lint warning on method overriding
 - Merge lworld
 - [bworld] Emit lint warnings if overriding method does not have same nullability
 - [bworld] Inference failure with bang
 - Fix RichDiagnosticFormatter not to strip metadata when computing where clauses
 - Merge lworld
 - Show null restriction markers in javadoc.
 - [bworld] Fix iterating arg types for null checks
 - [bworld] Remove the old NullRestricted attribute
 - ... and 130 more: https://git.openjdk.org/valhalla/compare/7ec54d5f...ec3737b7

The merge commit only contains trivial merges, so no merge-specific webrevs have been generated.

Changes: https://git.openjdk.org/valhalla/pull/2151/files
  Stats: 8198 lines in 123 files changed: 7706 ins; 152 del; 340 mod
  Patch: https://git.openjdk.org/valhalla/pull/2151.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2151/head:pull/2151

PR: https://git.openjdk.org/valhalla/pull/2151