Fix type inhabitedness check for arrays

Arrays of uninhabited types were considered to also be uninhabited if
their length had not been evaluated, causing unsoundness.

src/test/compile-fail/empty-never-array.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.
+
+#![feature(never_type)]
+
+enum Helper<T, U> {
+    T(T, [!; 0]),
+    #[allow(dead_code)]
+    U(U),
+}
+
+fn transmute<T, U>(t: T) -> U {
+    let Helper::U(u) = Helper::T(t, []); //~ ERROR refutable pattern in local binding: `T(_, _)` not covered
+    u
+}
+
+fn main() {
+    println!("{:?}", transmute::<&str, (*const u8, u64)>("type safety"));
+}