Fix unused_result lint triggering when a function returns () or !

Add a test for this case

src/librustc_lint/unused.rs

@@ -146,6 +146,8 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnusedResults {
 
         let t = cx.tables.expr_ty(&expr);
         let ty_warned = match t.sty {
+            ty::TyTuple(ref tys, _) if tys.is_empty() => return,
+            ty::TyNever => return,
             ty::TyAdt(def, _) => check_must_use(cx, def.did, s.span, ""),
             _ => false,
         };

src/test/ui/issue-43806.rs

@@ -0,0 +1,26 @@
+// 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.
+
+// run-pass
+
+#![deny(unused_results)]
+
+fn foo() {}
+
+fn bar() -> ! {
+    loop {}
+}
+
+fn qux() {
+    foo();
+    bar();
+}
+
+fn main() {}