Activate the ability to use expressions in indexes in a query. There are some

test failures, but mostly this seems to work.

FossilOrigin-Name: 42f93f582eccd8a778189aa6c113874f995ab751

src/whereInt.h

@@ -286,6 +286,7 @@ struct WhereScan {
   WhereClause *pOrigWC;      /* Original, innermost WhereClause */
   WhereClause *pWC;          /* WhereClause currently being scanned */
   char *zCollName;           /* Required collating sequence, if not NULL */
+  Expr *pIdxExpr;            /* Search for this index expression */
   char idxaff;               /* Must match this affinity, if zCollName!=NULL */
   unsigned char nEquiv;      /* Number of entries in aEquiv[] */
   unsigned char iEquiv;      /* Next unused slot in aEquiv[] */