[SPARK-11436] [SQL] rebind right encoder when join 2 datasets

When we join 2 datasets, we will combine 2 encoders into a tupled one, and use it as the encoder for the jioned dataset. Assume both of the 2 encoders are flat, their `constructExpression`s both reference to the first element of input row. However, when we combine 2 encoders, the schema of input row changed,  now the right encoder should reference to second element of input row. So we should rebind right encoder to let it know the new schema of input row before combine it.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #9391 from cloud-fan/join and squashes the following commits:

846d3ab [Wenchen Fan] rebind right encoder when join 2 datasets

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