parser: add test for 'extern crate async'

Author: Centril

src/test/ui/parser/extern-crate-async.rs

@@ -0,0 +1,12 @@
+// Make sure that we don't parse `extern crate async`
+// the front matter of a function leading us astray.
+
+// check-pass
+
+fn main() {}
+
+#[cfg(FALSE)]
+extern crate async;
+
+#[cfg(FALSE)]
+extern crate async as something_else;