add test for validation finding use-after-free

Author: RalfJung

tests/compile-fail/validity/dangling_ref3.rs

@@ -0,0 +1,10 @@
+use std::mem;
+
+fn dangling() -> *const u8 {
+    let x = 0u8;
+    &x as *const _
+}
+
+fn main() {
+    let _x: &i32 = unsafe { mem::transmute(dangling()) }; //~ ERROR dangling reference (use-after-free)
+}