RFR: 8246778: Compiler implementation for Sealed Classes (Second Preview) [v5]

[Jan Lahoda]

> This pull request replaces https://github.com/openjdk/jdk/pull/1227.
> 
> From the original PR:
> 
>> 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
>> 
>>     * renaming Class::permittedSubclasses to Class::getPermittedSubclasses, still in the same method, the return type has been changed to Class<?>[] instead of the previous ClassDesc[]
>> 
>>     * adding code to make sure that annotations can't be sealed
>> 
>>     * improving some tests
>> 
>> 
>> TIA
>> 
>> Related specs:
>> [Sealed Classes JSL](http://cr.openjdk.java.net/~gbierman/jep397/jep397-20201104/specs/sealed-classes-jls.html)
>> [Sealed Classes JVMS](http://cr.openjdk.java.net/~gbierman/jep397/jep397-20201104/specs/sealed-classes-jvms.html)
>> [Additional: Contextual Keywords](http://cr.openjdk.java.net/~gbierman/jep397/jep397-20201104/specs/contextual-keywords-jls.html)
> 
> This PR strives to reflect the review comments from 1227:
>  * adjustments to javadoc of j.l.Class methods
>  * package access checks in Class.getPermittedSubclasses()
>  * fixed to the narrowing conversion/castability as pointed out by Maurizio

Jan Lahoda has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  Cleanup as suggested on the review.

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1483/files
  - new: https://git.openjdk.java.net/jdk/pull/1483/files/537e267f..09e0c186

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1483&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1483&range=03-04

  Stats: 187 lines in 7 files changed: 38 ins; 114 del; 35 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1483.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1483/head:pull/1483

PR: https://git.openjdk.java.net/jdk/pull/1483