add test for warnings emitted when missing consuming attribute

Author: ktoso

test/Concurrency/custom_executor_enqueue_impls.swift

@@ -51,3 +51,15 @@ final class NewExecutor: SerialExecutor {
     UnownedSerialExecutor(ordinary: self)
   }
 }
+
+// Good impl, but missing the ownership keyword
+final class MissingOwnership: SerialExecutor {
+  func enqueue(_ job: ExecutorJob) {} // expected-error{{noncopyable parameter must specify its ownership}}
+  // expected-note@-1{{add 'borrowing' for an immutable reference}}
+  // expected-note@-2{{add 'inout' for a mutable reference}}
+  // expected-note@-3{{add 'consuming' to take the value from the caller}}
+
+  func asUnownedSerialExecutor() -> UnownedSerialExecutor {
+    UnownedSerialExecutor(ordinary: self)
+  }
+}