replace usages of fn_sig query with bound_fn_sig

src/main_shim.rs

@@ -46,7 +46,7 @@ fn create_entry_fn(
         is_main_fn: bool,
         sigpipe: u8,
     ) {
-        let main_ret_ty = tcx.fn_sig(rust_main_def_id).output();
+        let main_ret_ty = tcx.bound_fn_sig(rust_main_def_id).subst_identity().output();
         // Given that `main()` has no arguments,
         // then its return type cannot have
         // late-bound regions, since late-bound