Add unit test for case that didn't seem to be covered in existing UI tests

(since I made this mistake at first but the tests didn't catch it):
we should not suggest adding `mut` to a reassigned `ref` or `ref mut` binding.

(The Rust language, since at least 1.0, does not have `mut ref mut` or
`ref mut mut` etc.)

src/test/ui/reassign-ref-mut.rs

@@ -0,0 +1,26 @@
+// Copyright 2017 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.
+
+// Tests how we behave when the user attempts to mutate an immutable
+// binding that was introduced by either `ref` or `ref mut`
+// patterns.
+//
+// Such bindings cannot be made mutable via the mere addition of the
+// `mut` keyword, and thus we want to check that the compiler does not
+// suggest doing so.
+
+fn main() {
+    let (mut one_two, mut three_four) = ((1, 2), (3, 4));
+    let &mut (ref a, ref mut b) = &mut one_two;
+    a = &three_four.0;
+    //~^ ERROR cannot assign twice to immutable variable `a` [E0384]
+    b = &mut three_four.1;
+    //~^ ERROR cannot assign twice to immutable variable `b` [E0384]
+}

src/test/ui/reassign-ref-mut.stderr

@@ -0,0 +1,20 @@
+error[E0384]: cannot assign twice to immutable variable `a`
+  --> $DIR/reassign-ref-mut.rs:22:5
+   |
+LL |     let &mut (ref a, ref mut b) = &mut one_two;
+   |               ----- first assignment to `a`
+LL |     a = &three_four.0;
+   |     ^^^^^^^^^^^^^^^^^ cannot assign twice to immutable variable
+
+error[E0384]: cannot assign twice to immutable variable `b`
+  --> $DIR/reassign-ref-mut.rs:24:5
+   |
+LL |     let &mut (ref a, ref mut b) = &mut one_two;
+   |                      --------- first assignment to `b`
+...
+LL |     b = &mut three_four.1;
+   |     ^^^^^^^^^^^^^^^^^^^^^ cannot assign twice to immutable variable
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0384`.