RFR: Merge lworld

[Vicente Romero]

Merge lworld into universal-tvars

# Conflicts:
#	src/jdk.compiler/share/classes/com/sun/tools/javac/code/Source.java
#	src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Attr.java
#	src/jdk.compiler/share/classes/com/sun/tools/javac/comp/AttrContext.java
#	src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Flow.java
#	src/jdk.compiler/share/classes/com/sun/tools/javac/resources/compiler.properties

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Commit messages:
 - Merge branch 'lworld' into universal-tvars_merge_lworld_last_one
 - Merge lworld
 - left-overs from last type system refactoring
 - Merge lworld
 - addressing several issues in the implementation of universal tvars
 - Merge lworld
 - Merge lworld
 - changing universal lint warning to unchecked warning
 - Merge lworld
 - Build failing in universal tvars
 - ... and 22 more: https://git.openjdk.org/valhalla/compare/0eef4b51...d6c5956b

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

Changes: https://git.openjdk.org/valhalla/pull/747/files
  Stats: 1621 lines in 37 files changed: 1477 ins; 30 del; 114 mod
  Patch: https://git.openjdk.org/valhalla/pull/747.diff
  Fetch: git fetch https://git.openjdk.org/valhalla pull/747/head:pull/747

PR: https://git.openjdk.org/valhalla/pull/747