Add privately uninhabited dead code test

src/test/ui/uninhabited/privately-uninhabited-dead-code.rs

@@ -0,0 +1,19 @@
+// compile-pass
+
+#![deny(unreachable_code)]
+
+mod foo {
+    enum Bar {}
+
+    #[allow(dead_code)]
+    pub struct Foo {
+        value: Bar, // "privately" uninhabited
+    }
+
+    pub fn give_foo() -> Foo { panic!() }
+}
+
+fn main() {
+    foo::give_foo();
+    println!("Hello, world!"); // ok: we can't tell that this code is dead
+}