Improvements to distinct aggregates such that they can sometimes avoid

using an ephermeral table to test for duplicates if the column that is
distinct is part of an index.

FossilOrigin-Name: ef4ac0ddd297bbd38351410c5a387e1628561b3f1bec9e4c2c9d76fbe29f955a

src/sqliteInt.h

@@ -2603,6 +2603,7 @@ struct AggInfo {
     FuncDef *pFunc;          /* The aggregate function implementation */
     int iMem;                /* Memory location that acts as accumulator */
     int iDistinct;           /* Ephemeral table used to enforce DISTINCT */
+    int iDistAddr;           /* Address of OP_OpenEphemeral */
   } *aFunc;
   int nFunc;              /* Number of entries in aFunc[] */
   u32 selId;              /* Select to which this AggInfo belongs */