more flexible use of TREAT() in JOIN

Persistence issue #117 reported by @beikov requested support for things like:

```hql
SELECT a FROM BaseA a
LEFT JOIN FETCH TREAT(a AS A1).a1B
LEFT JOIN FETCH TREAT(a AS A2).a2B
```

This would be very useful.

Start by reproducing the HQL example in the issue and tracing how TREAT() expressions are handled in joins. No files or tests are named, so identify the relevant query parsing and persistence paths first; done means both TREAT() joins in the example are accepted and work as requested.

Written by the indexing model from the issue text.