Don't put 0-length array in the lltype of an arg-less variant

This seems to confuse LLVM in some very specific situations.

Closes rust-lang#883

src/comp/middle/trans_common.rs

@@ -696,7 +696,11 @@ fn T_opaque_closure_ptr(cx: &crate_ctxt) -> TypeRef {
 fn T_tag(tn: &type_names, size: uint) -> TypeRef {
     let s = "tag_" + uint::to_str(size, 10u);
     if tn.name_has_type(s) { ret tn.get_type(s); }
-    let t = T_struct([T_int(), T_array(T_i8(), size)]);
+    let t = if size == 0u {
+        T_struct([T_int()])
+    } else {
+        T_struct([T_int(), T_array(T_i8(), size)])
+    };
     tn.associate(s, t);
     ret t;
 }