core: Make intrinsics::init unsafe

as per #3920

src/libcore/unstable/intrinsics.rs

@@ -42,7 +42,10 @@ pub extern "rust-intrinsic" {
 
     pub fn get_tydesc<T>() -> *();
 
-    pub fn init<T>() -> T;
+    /// init is unsafe because it returns a zeroed-out datum,
+    /// which is unsafe unless T is POD. We don't have a POD
+    /// kind yet. (See #4074)
+    pub unsafe fn init<T>() -> T;
 
     #[cfg(not(stage0))]
     pub unsafe fn uninit<T>() -> T;