Tweak the raw_identifiers lints in 2018

* Enable the `raw_identifiers` feature automatically in the 2018 preview
* Only emit lint warnings if the `raw_identifiers` feature is activated

cc rust-lang/cargo#5783

src/test/ui/rust-2018/async-ident-allowed.rs

@@ -0,0 +1,21 @@
+// 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.
+
+// compile-flags: --edition 2015
+
+#![deny(rust_2018_compatibility)]
+
+// Don't make a suggestion for a raw identifer replacement unless raw
+// identifiers are enabled.
+
+fn main() {
+    let async = 3; //~ ERROR: is a keyword
+    //~^ WARN previously accepted
+}

src/test/ui/rust-2018/async-ident-allowed.stderr

@@ -0,0 +1,17 @@
+error: `async` is a keyword in the 2018 edition
+  --> $DIR/async-ident-allowed.rs:19:9
+   |
+LL |     let async = 3; //~ ERROR: is a keyword
+   |         ^^^^^
+   |
+note: lint level defined here
+  --> $DIR/async-ident-allowed.rs:13:9
+   |
+LL | #![deny(rust_2018_compatibility)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^
+   = note: #[deny(async_idents)] implied by #[deny(rust_2018_compatibility)]
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2018 edition!
+   = note: for more information, see issue #49716 <https://github.com/rust-lang/rust/issues/49716>
+
+error: aborting due to previous error
+