add arielby's example

tests/compile-fail/stacked_borrows/illegal_write6.rs

@@ -0,0 +1,12 @@
+fn main() {
+    let x = &mut 0u32;
+    let p = x as *mut u32;
+    foo(x, p);
+}
+
+fn foo(a: &mut u32, y: *mut u32) -> u32 {
+    *a = 1;
+    let _b = &*a;
+    unsafe { *y = 2; } //~ ERROR: borrow stack
+    return *a;
+}