Unsafe type cast in flexible parser QueryNodeImpl [LUCENE-7926]

`org.apache.lucene.queryparser.flexible.core.nodes.QueryNodeImplL67` casts the argument to `QueryNodeImpl` although the rest of the code is written against the `QueryNode` interface. This blows up in cases where a non-inheriting implementation is passed (such as using a wrapper/decorator class).

The relevant `setParent` method does not exist on the interface, so the most backwards compatible short term fix would be to wrap the cast in a conditional to ensure it is safe (more significant changes to the hierarchy may make sense but this seems like a fairly stable module).

Patch enclosed...

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Migrated from [LUCENE-7926](https://issues.apache.org/jira/browse/LUCENE-7926) by Matt Whipple
Attachments: [LUCENE-7926.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-7926/LUCENE-7926.patch)

Start with org.apache.lucene.queryparser.flexible.core.nodes.QueryNodeImpl and inspect the setParent method around the referenced line, then compare its argument handling with the QueryNode interface. Done means non-inheriting QueryNode implementations no longer trigger an unsafe cast while existing parent-setting behavior remains intact.

Written by the indexing model from the issue text.