QueryNode#cloneTree produces a new tree where parents are not correctly set [LUCENE-7266]

The following unit test performs a sanity check on the QueryNode tree, checking that each node has the parent set to the same node it was retrieved from. After calling cloneTree, this check fails on the returned node, as the parents in the cloned node still point back into the original tree.

```Java
import java.util.Arrays;
import java.util.List;

import org.apache.lucene.queryparser.flexible.core.nodes.BooleanQueryNode;
import org.apache.lucene.queryparser.flexible.core.nodes.FieldQueryNode;
import org.apache.lucene.queryparser.flexible.core.nodes.QueryNode;
import org.junit.Test;

public class TestCloneTree {
`@Test`
public void testCloneTree() throws Exception {
QueryNode original = new BooleanQueryNode(Arrays.asList(
new FieldQueryNode(null, "a", 0, 0),
new FieldQueryNode(null, "b", 0, 0)));

sanityCheckQueryTree(original);

QueryNode cloned = original.cloneTree();

sanityCheckQueryTree(cloned);
}

private void sanityCheckQueryTree(QueryNode node) {
List children = node.getChildren();
if (children != null) {
for (QueryNode child : children) {
// Matching what Lucene is using in QueryNodeImpl itself.
//noinspection ObjectEquality
if (child.getParent() != node) {
throw new IllegalStateException("Sanity check failed for child: " + child + '\n' +
" Parent is: " + child.getParent() + '\n' +
" But we got to it via: " + node);
}

sanityCheckQueryTree(child);
}
}
}
}
```

---
Migrated from [LUCENE-7266](https://issues.apache.org/jira/browse/LUCENE-7266) by Trejkaz

Start by reading QueryNode#cloneTree and the parent handling in QueryNodeImpl, then reproduce the sanity check shown in the issue with BooleanQueryNode and FieldQueryNode. Done means every child returned from the cloned tree has its parent set to the corresponding cloned node rather than to a node in the original tree.

Written by the indexing model from the issue text.