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

[Sean Perry via llvm-commits]

================
@@ -97,7 +101,9 @@ class thread {
     return *this;
   }
 
-  bool joinable() const noexcept { return Thread != native_handle_type(); }
+  bool is_null() const noexcept { return get_id() == 0; }
+
+  bool joinable() const noexcept { return !is_null(); }
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perry-ca wrote:

I discovered this code is built on windows.  I switched back to using `get_id() != 0`.

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