Change the name of SrcList.a[].zIndex to the more descriptive

SrcList.a[0].zIndexedBy.

FossilOrigin-Name: 6a9cf063379118dbb95c6cdc6d60af50e9867177

src/sqliteInt.h

@@ -2260,7 +2260,7 @@ struct SrcList {
     Expr *pOn;        /* The ON clause of a join */
     IdList *pUsing;   /* The USING clause of a join */
     Bitmask colUsed;  /* Bit N (1<<N) set if column N of pTab is used */
-    char *zIndex;     /* Identifier from "INDEXED BY <zIndex>" clause */
+    char *zIndexedBy; /* Identifier from "INDEXED BY <zIndex>" clause */
     Index *pIndex;    /* Index structure corresponding to zIndex, if any */
   } a[1];             /* One entry for each identifier on the list */
 };