Disallow dereferencing enum types when the variant is private

If an enum type's only variant is private, disallow dereferencing
values of its type.

Due to #4082, this only applies to enums that are in the same crate.

r=pcwalton

Closes #818

src/test/auxiliary/private_variant_1.rs

@@ -0,0 +1,5 @@
+mod super_sekrit {
+    pub enum sooper_sekrit {
+        pub quux, priv baz
+    }
+}

src/test/compile-fail/issue-818.rs

@@ -0,0 +1,14 @@
+mod ctr {
+
+    pub enum ctr { priv mkCtr(int) }
+
+    pub fn new(i: int) -> ctr { mkCtr(i) }
+    pub fn inc(c: ctr) -> ctr { mkCtr(*c + 1) }
+}
+
+
+fn main() {
+    let c = ctr::new(42);
+    let c2 = ctr::inc(c);
+    assert *c2 == 5; //~ ERROR can only dereference enums with a single, public variant
+}

src/test/compile-fail/private_variant_2.rs

@@ -0,0 +1,7 @@
+// xfail-test
+// aux-build:private_variant_1.rs
+extern mod private_variant_1;
+
+fn main() {
+    let _x = private_variant_1::super_sekrit::baz; //~ ERROR baz is private
+}