Record the proper typmod for an index expression column.

We should use exprTypmod() to extract the typmod of the expression,
instead of just blindly storing -1.  This seems to have been an aboriginal
oversight in commit fc8d970c which
introduced general-expression indexes.  The consequences are only cosmetic
at present, since the index machinery doesn't really look at typmod for
index columns; but still it seems best to describe the column type as
precisely as we can.  Per off-list complaint from Thomas Fanghaenel.

src/backend/catalog/index.c

@@ -389,7 +389,7 @@ ConstructTupleDescriptor(Relation heapRelation,
 			to->attalign = typeTup->typalign;
 			to->attstattarget = -1;
 			to->attcacheoff = -1;
-			to->atttypmod = -1;
+			to->atttypmod = exprTypmod(indexkey);
 			to->attislocal = true;
 			to->attcollation = collationObjectId[i];