[llvm] Enable using threads on z/OS (PR #171847)

[Sean Perry via llvm-commits]

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@@ -133,7 +139,7 @@ thread::thread(std::optional<unsigned> StackSizeInBytes, Function &&f,
 
   Thread = llvm_execute_on_thread_impl(ThreadProxy<CalleeTuple>, Callee.get(),
                                        StackSizeInBytes);
-  if (Thread != native_handle_type())
+  if (!is_null())
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perry-ca wrote:

sure.  I've made that change.

https://github.com/llvm/llvm-project/pull/171847