Back-port omit-unused-subquery-column enhancements into the 3.28 branch.

FossilOrigin-Name: 57a4e91f4343b612c346a4d284c0354deee364f83951e64b009e79cd20aedb47

src/sqliteInt.h

@@ -2677,6 +2677,7 @@ struct SrcList {
       unsigned isCorrelated :1;  /* True if sub-query is correlated */
       unsigned viaCoroutine :1;  /* Implemented as a co-routine */
       unsigned isRecursive :1;   /* True for recursive reference in WITH */
+      unsigned isCte :1;         /* This is a CTE */
     } fg;
     int iCursor;      /* The VDBE cursor number used to access this table */
     Expr *pOn;        /* The ON clause of a join */