Make univariant enums act like structs, so that they're aligned correctly.

Consts of such enums are aligned correctly, so we could either misalign
them to match the type_of, or fix the type_of.  The latter seems like a
better idea.

src/test/run-pass/const-enum-newtype-align.rs

@@ -0,0 +1,17 @@
+// Copyright 2013 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.
+
+enum E = u32;
+struct S { a: u8, b: E }
+const C: S = S { a: 0xA5, b: E(0xDEADBEEF) };
+
+fn main() {
+    assert C.b == 0xDEADBEEF;
+}