Fix assembler name collision

Gigi uses a dummy global variable to register global types for debug
info purposes and its name can now collide with user variables.

gcc/ada/ChangeLog:
	* gcc-interface/trans.c (lvalue_for_aggregate_p): Also return true
	for return statements.
	* gcc-interface/utils.c (gnat_write_global_declarations): Use the
	maximum index for the dummy object to avoid a name collision.

gcc/ada/gcc-interface/trans.c

@@ -970,6 +970,10 @@ lvalue_for_aggregate_p (Node_Id gnat_node, tree gnu_type)
       /* Even if the parameter is by copy, prefer an lvalue.  */
       return true;
 
+    case N_Simple_Return_Statement:
+      /* Likewise for a return value.  */
+      return true;
+
     case N_Indexed_Component:
     case N_Selected_Component:
       /* If an elementary component is used, take it from the constant.  */

gcc/ada/gcc-interface/utils.c

@@ -5903,7 +5903,7 @@ gnat_write_global_declarations (void)
       struct varpool_node *node;
       char *label;
 
-      ASM_FORMAT_PRIVATE_NAME (label, first_global_object_name, 0);
+      ASM_FORMAT_PRIVATE_NAME (label, first_global_object_name, ULONG_MAX);
       dummy_global
 	= build_decl (BUILTINS_LOCATION, VAR_DECL, get_identifier (label),
 		      void_type_node);