remove the "defaulted unit" type bit during writeback

The defaulted unit bit is only relevant for the surrounding inference
context, and can cause trouble, including spurious lints and ICEs,
outside of it.

Fixes #43853.

src/test/run-pass/issue-43853.rs

@@ -0,0 +1,24 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::panic;
+
+fn test() {
+    wait(|| panic!());
+}
+
+fn wait<T, F: FnOnce() -> T>(f: F) -> F::Output {
+    From::from(f())
+}
+
+fn main() {
+    let result = panic::catch_unwind(move || test());
+    assert!(result.is_err());
+}