Allow dataflow refinement of wildcard bounds

The checker fails to infer a non-null type for the wildcard in List<?> lx = ...:

import org.checkerframework.checker.nullness.qual.NonNull;
import java.util.List;

abstract class Test {
  abstract List<? extends @NonNull Object> getList();
  void f() {
    List<?> lx = getList();
    for (Object l : lx) {
      l.toString();
    }
  }
}

$ ~/jsr308/checker-framework-1.9.5/checker/bin/javac -processor org.checkerframework.checker.nullness.NullnessChecker -version Test.java
javac 1.8.0-jsr308-1.9.5
Test.java:9: error: [dereference.of.nullable] dereference of possibly-null reference l
      l.toString();
      ^
1 error

Start by reproducing the failure in Test.java with the NullnessChecker command shown in the issue. Trace how wildcard bounds and dataflow refinement are handled, then add a regression test showing that iterating List<?> obtained from List<? extends @NonNull Object> permits l.toString() without a nullable-reference diagnostic.