Adjust crash bug to still reproduce.

This test reproduces a rustc ICE. Unfortunately, the changes to lifetime
elision mask the original ICE bug by making this function signature
illegal. However, by simplifying the signature we can regain the
original ICE.

tests/crashes/122903-1.rs

@@ -1,7 +1,7 @@
 //@ known-bug: #122903
 impl Struct {
-    async fn box_box_ref_Struct(
-        self: Box<Box<Self, impl FnMut(&mut Box<Box<Self, impl FnMut(&mut Self)>>)>>,
+    fn box_box_ref_Struct(
+        self: impl FnMut(Box<impl FnMut(&mut Self)>),
     ) -> &u32 {
         f
     }