Update a test for the new add-predates-concurrency remark

(cherry picked from commit ceedb01)

Author: DougGregor

test/ClangImporter/objc_async.swift

@@ -4,6 +4,7 @@
 // REQUIRES: concurrency
 import Foundation
 import ObjCConcurrency
+// expected-remark@-1{{add '@_predatesConcurrency' to suppress 'Sendable'-related warnings from module 'ObjCConcurrency'}}
 
 if #available(SwiftStdlib 5.5, *) {