RFR: 8246778: Compiler implementation for Sealed Classes (Second Preview)

[Vicente Romero]

Please review the code for the second iteration of sealed classes. In this iteration we are:

- Enhancing narrowing reference conversion to allow for stricter checking of cast conversions with respect to sealed type hierarchies.
- Also local classes are not considered when determining implicitly declared permitted direct subclasses of a sealed class or sealed interface

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Commit messages:
 - 8246778: Compiler implementation for Sealed Classes (Second Preview)

Changes: https://git.openjdk.java.net/jdk/pull/1227/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1227&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8246778
  Stats: 589 lines in 12 files changed: 508 ins; 18 del; 63 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1227.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1227/head:pull/1227

PR: https://git.openjdk.java.net/jdk/pull/1227