Assign correct types to struct-like enum variant constructors

Before, the type was just the enum type itself, which caused an
assertion failure in iter_variant in trans::base.

r=brson

Closes #4229

src/test/run-pass/enum-variants.rs

@@ -0,0 +1,11 @@
+enum Animal {
+    Dog (~str, float),
+    Cat { name: ~str, weight: float }
+}
+
+fn main() {
+    let mut a: Animal = Dog(~"Cocoa", 37.2);
+    a = Cat{ name: ~"Spotty", weight: 2.7 };
+    // permuting the fields should work too
+    let c = Cat { weight: 3.1, name: ~"Spreckles" };
+}