Add run-pass test for reinitialized unions.

This commit adds a run-pass test for the subset of
`src/test/ui/borrowck/borrowck-union-move-assign.rs` that is intended to
pass as the union is reinitialized.

Author: davidtwco

Diff for: src/test/ui/nll/issue-55651.rs

@@ -0,0 +1,38 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-pass
+
+#![feature(untagged_unions)]
+
+struct A;
+struct B;
+
+union U {
+    a: A,
+    b: B,
+}
+
+fn main() {
+    unsafe {
+        {
+            let mut u = U { a: A };
+            let a = u.a;
+            u.a = A;
+            let a = u.a; // OK
+        }
+        {
+            let mut u = U { a: A };
+            let a = u.a;
+            u.b = B;
+            let a = u.a; // OK
+        }
+    }
+}