[Test] Mark unsupported for OS stdlib.

The test verifies a stdlib behavior whose fix is not present in all
OSes.

rdar://136943907

Author: nate-chandler

validation-test/SILOptimizer/rdar133969821.swift

@@ -4,6 +4,7 @@
 // REQUIRES: concurrency
 // REQUIRES: concurrency_runtime
 // UNSUPPORTED: back_deployment_runtime
+// UNSUPPORTED: use_os_stdlib
 
 class C {
   init() {}