Add a tracking issue for making the warning a lint

src/librustc_mir/interpret/step.rs

@@ -11,6 +11,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M> {
     pub fn inc_step_counter_and_check_limit(&mut self, n: usize) {
         self.terminators_remaining = self.terminators_remaining.saturating_sub(n);
         if self.terminators_remaining == 0 {
+            // FIXME(#49980): make this warning a lint
             self.tcx.sess.span_warn(self.frame().span, "Constant evaluating a complex constant, this might take some time");
             self.terminators_remaining = 1_000_000;
         }