Arbitrary self types v2: hacky fix

This "fix" indicates that the current approach around
`receiver_trait_derefs` may not be good enough for extension traits.
This needs more thought before we land this, but pushing this commit
meanwhile to see if this PR passes the CI in other ways.

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -768,6 +768,8 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             self.receiver_trait_derefs = step.autoderefs;
             self.assemble_probe(&step.self_ty);
         }
+        // FIXME this isn't good
+        self.receiver_trait_derefs = 0;
     }
 
     #[instrument(level = "debug", skip(self))]