Improving Java code completion support for sealed types in switches.

[lahodaj]

Consider code like:

package sealed.test;

public class SealedTest {
    private int test(I i) {
        return switch (i) {
            case /*invoke code completion here*/
        };
    }

    public sealed interface I {}
    public final class A implements I {}
    public final class B implements I {}
    public class E extends Exception {}
    public enum AI implements I {A, B, C;}
}

The (Java) code completion returns all types at the specified place, instead of just subtypes of i. This patch is trying to solve that, by using the selector's type as a base type, and hence only proposing the correct subclasses/interfaces.

The patch is sadly a bit biggish, partly as it seems I need to add smartType flag to createStaticMemberItem. The code is also tricky around qualified enum constants (which are permitted since JDK 21).

There may be further improvements possible, but I hope this PR leads to a not completely unreasonable state.