[clangd] Destroy context before resetting CurrentReq

[kadircet]

Summary

Our tests stash callbacks into request context and rely on it being
invoked before threads going idle.

clang-tools-extra/clangd/TUScheduler.cpp

while (true) {
         // Wait until stop is called or there is a request.
         ReqCV.wait(Lock, [this] { return NextReq || Done; });
         if (Done)
           break;
         CurrentReq = std::move(*NextReq);
         NextReq.reset();
       }
 
+      {
         WithContext Guard(std::move(CurrentReq->Ctx));
         // Build the preamble and let the waiters know about it.
         build(std::move(*CurrentReq));
+      }
       bool IsEmpty = false;
       {
         std::lock_guard<std::mutex> Lock(Mutex);
         CurrentReq.reset();
         IsEmpty = !NextReq.hasValue();
       }
       if (IsEmpty) {
         // We don't perform this above, before waiting for a request to make