address review comments

- move constraints to an Option
- check `-Zpolonius=next` only once
- rewrite fixme comments to make the actionable part clear

compiler/rustc_borrowck/src/lib.rs

@@ -322,7 +322,7 @@ fn do_mir_borrowck<'tcx>(
         body,
         &regioncx,
         &borrow_set,
-        &localized_outlives_constraints,
+        localized_outlives_constraints,
         &opt_closure_req,
     );