Fix wildcard nullability reasoning for fields and locals

Follow up to #1558.  E.g., we should report an error for:
```java
class Test {
  static class Foo<T extends @Nullable Object> {
    T get() { throw new RuntimeException(); }
  }
  static void testNullableExtendsBound(Foo<? extends @Nullable Object> f) {
    Object x = f.get();
    // report error here
    x.hashCode();
  }
}
```