RFR: 8274363: Transitively sealed classes not considered exhaustive in switches

[Jan Lahoda]

Consider code like:

public class SwitchCoverage {
    sealed interface A {}
    sealed interface B1 extends A {}
    sealed interface B2 extends A {}
    sealed interface C extends A {}
    final class D1 implements B1, C {}
    final class D2 implements B2, C {}
    
    void test(A arg) {
        int i = switch (arg) {
            case B1 b1 -> 1;
            case B2 b2 -> 2;
        };
    }
    
} 


Note that `B1` covers `D1` and `B2` covers `D2`. So, `B1` and `B2` cover `C`, and hence `B1` and `B2` cover `A`.  To detect this, when looking for coverage of permitted types of `A`, we need to transitively look if any of the permitted type is covered.

The check uses the `DeferredCompletionFailureHandler`, because if don't want to fail the compilation on unresolvable permitted subclass. (In case the permitted subclass is covered by some other means, of course.)

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

Commit messages:
 - Fixing tests.
 - 8274363: Transitively sealed classes not considered exhaustive in switches

Changes: https://git.openjdk.java.net/jdk/pull/5717/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5717&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274363
  Stats: 72 lines in 2 files changed: 54 ins; 8 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5717.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5717/head:pull/5717

PR: https://git.openjdk.java.net/jdk/pull/5717