Refinement lost for switch expressions with yields.

[smillst]

The following code should type check without any errors.

import java.util.function.Supplier;
import org.checkerframework.checker.nullness.qual.NonNull;

public final class IssueXXX {
  enum TestEnum { FIRST, SECOND };

  public static void main(String[] args) {
    TestEnum testEnum = TestEnum.FIRST;
 Supplier<Integer> supplier = switch (testEnum) {
      case FIRST: yield () -> 1;
      case SECOND: yield() -> 2;
    };
   @NonNull Supplier<Integer> supplier1 = supplier;
  }
}

../testcases/src/Issue5930.java:13: error: [assignment] incompatible types in assignment.
   @NonNull Supplier<Integer> supplier1 = supplier;
                                          ^
  found   : @UnknownInitialization @Nullable Supplier<@Initialized @NonNull Integer>
  required: @UnknownInitialization @NonNull Supplier<@Initialized @NonNull Integer>