Added compile-fail test for $#metavar hygiene opt-out syntax.

We want this to fail for the time being, per <#47992 (comment)>.

Author: Alexander Regueiro

src/test/compile-fail/hygiene-optout-5.rs

@@ -0,0 +1,24 @@
+// Copyright 2018 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.
+
+#![feature(decl_macro)]
+
+macro m($mod_name:ident) {
+    pub mod $#mod_name {
+    //~^ ERROR expected identifier, found `#`
+    //~| ERROR unknown macro variable ``
+    //~| ERROR expected identifier, found reserved identifier ``
+    //~| ERROR expected one of `;` or `{`, found `mod_name`
+    }
+}
+
+fn main() {
+    m!(foo);
+}