RFR: 8246778: Compiler implementation for Sealed Classes (Second Preview) [v6]

[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 updated the pull request incrementally with one additional 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/09e0c186..b5304057

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1483&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1483&range=04-05

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 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