## What changes were proposed in this pull request?
The result of a `sum` aggregate function is typically a Decimal, Double or a Long. Currently the output dataType is based on input's dataType.
The `FunctionArgumentConversion` rule will make sure that the input is promoted to the largest type, and that also ensures that the output uses a (hopefully) sufficiently large output dataType. The issue is that sum is in a resolved state when we cast the input type, this means that rules assuming that the dataType of the expression does not change anymore could have been applied in the mean time. This is what happens if we apply `WidenSetOperationTypes` before applying the casts, and this breaks analysis.
The most straight forward and future proof solution is to make `sum` always output the widest dataType in its class (Long for IntegralTypes, Decimal for DecimalTypes & Double for FloatType and DoubleType). This PR implements that solution.
We should move expression specific type casting rules into the given Expression at some point.
## How was this patch tested?
Added (regression) tests to SQLQueryTestSuite's `union.sql`.
Author: Herman van Hovell <hvanhovell@databricks.com>
Closes#16063 from hvanhovell/SPARK-18622.
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