Minor. Additional test for "async {}"

compiler/testData/diagnostics/tests/ReservedAsync.kt

@@ -6,6 +6,7 @@ infix fun Any.async(f: () -> Unit) = f()
 
 fun test(foo: Any) {
     <!UNSUPPORTED!>async<!> {  }
+    `async` {  }
     <!UNSUPPORTED!>async<!> /**/ {  }
     foo <!UNSUPPORTED!>async<!> {  }
 
@@ -15,6 +16,7 @@ fun test(foo: Any) {
     foo async ({ })
 
     foo <!UNSUPPORTED!>async<!> fun () {}
+    foo `async` fun () {}
     foo async (fun () {})
 
     async (fun () {})