First cut at logic to perform DO UPDATE for rowid tables.

FossilOrigin-Name: a9080bc8b8c5f3b399eb1819bb5009581f178d85bb2b2cca7bc16a7b81b06863

src/sqliteInt.h

@@ -2728,6 +2728,7 @@ struct Upsert {
   Index *pUpsertIdx;        /* Constraint that pUpsertTarget identifies */
   ExprList *pUpsertSet;     /* The SET clause from an ON CONFLICT UPDATE */
   Expr *pUpsertWhere;       /* WHERE clause for the ON CONFLICT UPDATE */
+  SrcList *pUpsertSrc;      /* Table to be updated */
 };
 
 /*