Perf/debug: Add debug info to ListComprehensionRootNode

Add proper toString method so that we can identify those root nodes in
logs and in the Ideal Graph Visualizer

Author: raph-amiard

lkql_jit/language/src/main/java/com/adacore/lkql_jit/nodes/root_nodes/ListComprehensionRootNode.java

@@ -73,4 +73,14 @@ public Object execute(VirtualFrame frame) {
             return null;
         }
     }
+
+    @Override
+    public String toString() {
+        return (
+            "<listcomp>:" +
+            this.result.getLocation().fileName() +
+            ":" +
+            this.result.getLocation().startLine()
+        );
+    }
 }