ada: Handle unchecked conversion in bound

Look through both unchecked and normal conversions when seeing if any
part of a bound is uplevel.

gcc/ada/

	* exp_unst.adb (Note_Uplevel_Bound): Treat
	N_Unchecked_Type_Conversion like N_Type_Conversion.

gcc/ada/exp_unst.adb

@@ -643,7 +643,9 @@ package body Exp_Unst is
 
                   --  Conversion case
 
-                  elsif Nkind (N) = N_Type_Conversion then
+                  elsif Nkind (N) in
+                          N_Type_Conversion | N_Unchecked_Type_Conversion
+                  then
                      Note_Uplevel_Bound (Expression (N), Ref);
                   end if;
                end Note_Uplevel_Bound;