[SPARK-17760][SQL] AnalysisException with dataframe pivot when groupBy column is not attribute

## What changes were proposed in this pull request?

Fixes AnalysisException for pivot queries that have group by columns that are expressions and not attributes by substituting the expressions output attribute in the second aggregation and final projection.

## How was this patch tested?

existing and additional unit tests

Author: Andrew Ray <ray.andrew@gmail.com>

Closes #16177 from aray/SPARK-17760.

(cherry picked from commit f1fca81b)
Signed-off-by: Herman van Hovell <hvanhovell@databricks.com>