Add feature gate tests

As part of the audit for #22820 the following feature gate tests have
been added:

* `rustc_diagnostic_macros`

/cc #19624

Author: lgrz

src/test/compile-fail/feature-gate-rustc-diagnostic-macros.rs

@@ -0,0 +1,23 @@
+// Copyright 2015 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.
+
+// Test that diagnostic macros are gated by `rustc_diagnostic_macros` feature
+// gate
+
+__register_diagnostic!(E0001);
+//~^ ERROR macro undefined: '__register_diagnostic!'
+
+fn main() {
+    __diagnostic_used!(E0001);
+    //~^ ERROR macro undefined: '__diagnostic_used!'
+}
+
+__build_diagnostic_array!(DIAGNOSTICS);
+//~^ ERROR macro undefined: '__build_diagnostic_array!'