Recompute the set of columns used for each table when the table is

involved in query flattening.

FossilOrigin-Name: a9bb71ba708ba72255ba8d18c9856e38ddf53eae2d61c8435149354fb2b2459e

src/sqliteInt.h

@@ -3695,6 +3695,7 @@ struct Walker {
     struct WhereConst *pConst;                /* WHERE clause constants */
     struct RenameCtx *pRename;                /* RENAME COLUMN context */
     struct Table *pTab;                       /* Table of generated column */
+    struct SrcList_item *pSrcItem;            /* A single FROM clause item */
   } u;
 };