make const_err a future incompat lint

src/test/ui/consts/const-eval/conditional_array_execution.rs

@@ -6,9 +6,11 @@ const X: u32 = 5;
 const Y: u32 = 6;
 const FOO: u32 = [X - Y, Y - X][(X < Y) as usize];
 //~^ WARN any use of this value will cause an error
+//~| WARN this was previously accepted by the compiler but is being phased out
 
 fn main() {
     println!("{}", FOO);
     //~^ ERROR
     //~| WARN erroneous constant used [const_err]
+    //~| WARN this was previously accepted by the compiler but is being phased out
 }