HPCC-20708 Ensure index filenames are correctly expanded by BUILD

If the name of an index used in a build statement was dependent on
__TARGET_PLATFORM__ the compiler could fail with an internal error.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>

ecl/hqlcpp/hqlttcpp.cpp

@@ -2134,7 +2134,7 @@ static IHqlExpression * normalizeIndexBuild(IHqlExpression * expr, bool sortInde
             args.remove(i1);
             args.append(*createAttribute(sortedAtom));
             args.append(*createLocalAttribute());
-            args.append(*createAttribute(indexAtom, LINK(index->queryChild(3))));
+            args.append(*createExprAttribute(indexAtom, LINK(index->queryChild(3))));
             return expr->clone(args);
         }
     }