test: Add a simple test for dangling pointers

Author: pcwalton

src/test/compile-fail/regions-ret.rs

@@ -0,0 +1,9 @@
+// error-pattern: mismatched types
+
+fn f(x : &a.int) -> &a.int {
+    ret &3;
+}
+
+fn main() {
+}
+