[clangd] Fix race in Global CDB shutdown

I believe the atomic write can be reordered after the notify, and that
seems to be happening on mac m1: http://45.33.8.238/macm1/2654/step_8.txt
In practice maybe seq_cst is enough? But no reason not to lock here.

https://bugs.llvm.org/show_bug.cgi?id=48998

clang-tools-extra/clangd/GlobalCompilationDatabase.cpp

@@ -34,6 +34,7 @@
 #include <atomic>
 #include <chrono>
 #include <condition_variable>
+#include <mutex>
 #include <string>
 #include <tuple>
 #include <vector>
@@ -553,7 +554,10 @@ public:
   }
 
   ~BroadcastThread() {
-    ShouldStop.store(true, std::memory_order_release);
+    {
+      std::lock_guard<std::mutex> Lock(Mu);
+      ShouldStop.store(true, std::memory_order_release);
+    }
     CV.notify_all();
     Thread.join();
   }