Add some more tests involving Offset/arith_offset and ZST pointers

tests/run-pass/drop_empty_slice.rs

@@ -0,0 +1,9 @@
+#![feature(box_syntax)]
+// This disables the test completely:
+// ignore-stage1
+
+fn main() {
+    // With the nested Vec, this is calling Offset(Unique::empty(), 0).
+    let args : Vec<Vec<i32>> = Vec::new();
+    let local = box args;
+}

tests/run-pass/iter_slice.rs

@@ -0,0 +1,12 @@
+fn main() {
+    for _ in Vec::<u32>::new().iter() { // this iterates over a Unique::empty()
+        panic!("We should never be here.");
+    }
+
+    // Iterate over a ZST (uses arith_offset internally)
+    let mut count = 0;
+    for _ in &[(), (), ()] {
+        count += 1;
+    }
+    assert_eq!(count, 3);
+}