[llvm] Fix AppVerifier GetThreadId(NULL) error in llvm::thread (PR #216941)

[Saleem Abdulrasool via llvm-commits]

================
@@ -49,7 +49,15 @@ void llvm_thread_detach_impl(HANDLE hThread) {
     ReportLastErrorFatal("CloseHandle failed");
 }
 
-DWORD llvm_thread_get_id_impl(HANDLE hThread) { return ::GetThreadId(hThread); }
+DWORD llvm_thread_get_id_impl(HANDLE hThread) {
+  // AppVerifier (runtime verification tool from Windows SDK) reports an error
+  // when GetThreadId(NULL) is called. Suppress this by manually checking for
+  // NULL. Return 0 as this is what's returned (by documentation) if
+  // GetThreadId() fails.
+  if (!hThread)
+    return 0;
+  return ::GetThreadId(hThread);
----------------
compnerd wrote:

Why not write this as:

```
if (hThread)
  return ::GetThreadId(hThread);
return ::GetThreadId(::GetCurrentThread());
```

That should preserve the semantics desired given the description in the commit message that it is used to get the current thread. The current thread psuedo handle is `(HANDLE)-2`.

https://github.com/llvm/llvm-project/pull/216941