Missed l-value calls to future::then in Benchmark.cpp

Summary:
Overall plan to modify Future<T>::then to be r-value qualified and use Future<T>::thenTry or Future<T>::thenValue. The goal is to disambiguate folly::Future and to improve type and lifetime safety of Future and its methods.

Add a std::move that was missed in the folly benchmark.

Reviewed By: shixiao

Differential Revision: D9197265

fbshipit-source-id: d02ab7b0dc22f6465910a80b2d0af1b54fb7bde7

folly/futures/test/Benchmark.cpp

@@ -36,7 +36,7 @@ T incr(Try<T>&& t) {
 void someThens(size_t n) {
   auto f = makeFuture<int>(42);
   for (size_t i = 0; i < n; i++) {
-    f = f.then(incr<int>);
+    f = std::move(f).then(incr<int>);
   }
 }
 
@@ -101,7 +101,7 @@ BENCHMARK(no_contention) {
     consumer = std::thread([&]{
       b1.post();
       for (auto& f : futures) {
-        f.then(incr<int>);
+        std::move(f).then(incr<int>);
       }
     });
     consumer.join();
@@ -138,7 +138,7 @@ BENCHMARK_RELATIVE(contention) {
       b1.post();
       for (auto& f : futures) {
         sem_wait(&sem);
-        f.then(incr<int>);
+        std::move(f).then(incr<int>);
       }
     });