Alternative fix to ticket [c8d3b9f0a750a529]: Prior to deleting or modifying

an Expr not that is referenced by an AggInfo, modify the AggInfo to get its
own copy of the original Expr.

FossilOrigin-Name: 7682d8a768fbccfe0cc956e9f6481637146e1ab9763b248ff11052761ce32e32