[clang-tools-extra] [NFC] Avoid data race condition. (PR #118664)

[Zahira Ammarguellat via cfe-commits]

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@@ -501,6 +501,7 @@ class PreambleThread {
       }
 
       {
+        std::unique_lock<std::mutex> Lock(Mutex);
         WithContext Guard(std::move(CurrentReq->Ctx));
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zahiraam wrote:

Thanks @tahonermann. I had forgotten about it. Closing it.

https://github.com/llvm/llvm-project/pull/118664