cql3: select_statement: force aggregation if GROUP BY is used

GROUP BY is typically used with aggregation. In one case the aggregation
is implicit:

    SELECT a, b, c
    FROM tab
    GROUP BY x, y, z

One row will appear from each group, even though no aggregation
was specified. To avoid this irregularity, rewrite this query as

    SELECT first(a), first(b), first(c)
    FROM tab
    GROUP BY x, y, z

This allows us to have different paths for aggregations and
non-aggregations, without worrying about this special case.

cql3/statements/select_statement.cc

@@ -1699,6 +1699,12 @@ std::unique_ptr<prepared_statement> select_statement::prepare(data_dictionary::d
     prepared_selectors = maybe_jsonize_select_clause(std::move(prepared_selectors), db, schema);
 
     auto aggregation_depth = 0u;
+
+    // Force aggregation if GROUP BY is used. This will wrap every column x as first(x).
+    if (!_group_by_columns.empty()) {
+        aggregation_depth = std::max(aggregation_depth, 1u);
+    }
+
     for (auto& ps : prepared_selectors) {
         aggregation_depth = std::max(aggregation_depth, expr::aggregation_depth(ps.expr));
     }