[clangd] Fix a race

[sammccall]

Thanks!

clang-tools-extra/clangd/unittests/ClangdLSPServerTests.cpp

void stop() override {
       CV.notify_all();
     }
 
   public:
     AsyncCounter() : Thread([this] { run(); }) {}
     ~AsyncCounter() {
       // Verify shutdown sequence was performed.
       // Real modules would not do this, to be robust to no ClangdServer.
+      {
+        // We still need the lock here, as Queue might be empty when
+        // ClangdServer calls blockUntilIdle, but run() might not have returned
+        // yet.
+        std::lock_guard<std::mutex> Lock(Mu);
         EXPECT_TRUE(ShouldStop) << "ClangdServer should request shutdown";
         EXPECT_EQ(Queue.size(), 0u) << "ClangdServer should block until idle";
+      }
       Thread.join();
     }
 
     void initializeLSP(LSPBinder &Bind, const llvm::json::Object &ClientCaps,
                        llvm::json::Object &ServerCaps) override {
       Bind.notification("increment", this, &AsyncCounter::increment);
     }