Fix -Wpessimizing-move warnings

[stephan.dollberg]

Summary

Fixes a bunch of -Wpessimizing-move warnings as warned by gcc9.
std::moves in returns which prevents copy elision.

[stephan.dollberg]

Rebased

[lhames]

LGTM. Please go ahead and commit. If you do not have commit access just let me know and I can commit on your behalf.

[stephan.dollberg]

Yes please that would be great, I don't have access.

[stephan.dollberg]

Rebased again, resolved conflicts

llvm/include/llvm/ExecutionEngine/Orc/RPC/RPCUtils.h

if (auto Err = this->template appendCallAsync<Func>(
             [&](ResultType R) {
               Result = std::move(R);
               ReceivedResponse = true;
               return Error::success();
             },
             Args...)) {
       detail::ResultTraits<typename Func::ReturnType>::consumeAbandoned(
           std::move(Result));
-      return std::move(Err);
+      return Err;
     }
 
     if (auto Err = this->C.send()) {
       detail::ResultTraits<typename Func::ReturnType>::consumeAbandoned(
           std::move(Result));
-      return std::move(Err);
+      return Err;
     }
 
     while (!ReceivedResponse) {
       if (auto Err = this->handleOne()) {
         detail::ResultTraits<typename Func::ReturnType>::consumeAbandoned(
             std::move(Result));
-        return std::move(Err);
+        return Err;
       }
     }
 
     return Result;
   }
 };
 
 /// Asynchronous dispatch for a function on an RPC endpoint.

llvm/include/llvm/Support/TaskQueue.h

std::future<ResultTy> F = T.P->get_future();
       // task immediately.
       if (IsTaskInFlight)
         Tasks.push_back(std::move(T));
       else {
         Scheduler.async(std::move(T));
         IsTaskInFlight = true;
       }
     }
-    return std::move(F);
+    return F;
   }
 
 private:
   void completeTask() {
     // We just completed a task.  If there are no more tasks in the queue,
     // update IsTaskInFlight to false and stop doing work.  Otherwise
     // schedule the next task (while not holding the lock).
     std::function<void()> Continuation;