Fix issue with &&mut T

```
error[E0596]: cannot borrow `*self.infcx` as mutable, as it is behind a `&` reference
   --> compiler/rustc_infer/src/infer/canonical/query_response.rs:642:9
    |
641 |     fn infcx(&self) -> &mut InferCtxt<'cx, 'tcx> {
    |              ----- help: consider changing this to be a mutable reference: `&mut self`
642 |         self.infcx
    |         ^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable
```

Author: jyn514

compiler/rustc_infer/src/infer/canonical/query_response.rs

@@ -638,7 +638,7 @@ struct QueryTypeRelatingDelegate<'a, 'cx, 'tcx> {
 }
 
 impl<'a, 'cx, 'tcx> TypeRelatingDelegate<'cx, 'tcx> for QueryTypeRelatingDelegate<'a, 'cx, 'tcx> {
-    fn infcx(&self) -> &mut InferCtxt<'cx, 'tcx> {
+    fn infcx(&mut self) -> &mut InferCtxt<'cx, 'tcx> {
         self.infcx
     }
 

compiler/rustc_infer/src/infer/nll_relate/mod.rs

@@ -72,7 +72,7 @@ where
 }
 
 pub trait TypeRelatingDelegate<'cx, 'tcx> {
-    fn infcx(&self) -> &mut InferCtxt<'cx, 'tcx>;
+    fn infcx(&mut self) -> &mut InferCtxt<'cx, 'tcx>;
 
     /// Push a constraint `sup: sub` -- this constraint must be
     /// satisfied for the two types to be related. `sub` and `sup` may