Fix the inline varchar serial aggregate group by wrong answer bug

Waiting for Jenkins...
I wonder if the storage swap (and the similar one for partial aggregation) that happens here could be factored into a method in the base class.  The subclasses could call this method.

I found it a little confusing that we're swapping storage for m_inProgressGroupByKeyTuple (member of subclass) and m_nextGroupByKeyStorage (member of base class).