Add run-pass test for 2018 edition code using 2015 edition macros with edition-gated keywords

Author: Manishearth

src/test/run-pass/auxiliary/edition-kw-macro-2015.rs

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+// 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: -Zedition=2015 -Zunstable-options
+#![feature(raw_identifiers)]
+
+#[macro_export]
+macro_rules! produces_async {
+    () => (pub fn async() {})
+}
+
+#[macro_export]
+macro_rules! produces_async_raw {
+    () => (pub fn r#async() {})
+}
+
+#[macro_export]
+macro_rules! consumes_async {
+    (async) => (1)
+}
+
+#[macro_export]
+macro_rules! consumes_async_raw {
+    (r#async) => (1)
+}
+
+#[macro_export]
+macro_rules! consumes_ident {
+    ($i:ident) => ($i)
+}

src/test/run-pass/edition-keywords-2018.rs

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+// 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: -Zedition=2018 -Zunstable-options
+// aux-build:edition-kw-macro-2015.rs
+
+#![feature(raw_identifiers)]
+
+#[macro_use]
+extern crate edition_kw_macro_2015;
+
+pub fn main() {
+    let mut r#async = 1;
+
+    r#async = consumes_async!(r#async);
+    r#async = consumes_async_raw!(r#async);
+
+    if consumes_ident!(r#async) == 1 {
+        // ...
+    }
+    one::r#async();
+    two::r#async();
+}
+
+
+mod one {
+    produces_async! {}
+}
+mod two {
+    produces_async_raw! {}
+}