Temporarily turn off async test for iphonesimulator-x86_64.

This may be related to the workaround in PR swiftlang#37939.
See also: rdar://79670222.

Author: varungandhi-apple

test/Concurrency/Runtime/async.swift

@@ -7,6 +7,9 @@
 // rdar://76038845
 // UNSUPPORTED: use_os_stdlib
 
+// rdar://79670222 : This test fails on iphonesimulator-x86_64
+// UNSUPPORTED: OS=ios && CPU=x86_64
+
 import Dispatch
 import StdlibUnittest