use rvalue-qual Future::get()

Summary:
This is part of "the great r-valuification of folly::Future":

* This is something we should do for safety in general.
* Using lvalue-qualified `Future::get()` has caused some failures around D7840699 since lvalue-qualification hides that operation's move-out semantics - leads to some use of future operations that are really not correct, but are not obviously incorrect.
* Problems with `Future::get() &`: it moves-out the result but doesn't invalidate the Future - the Future remains (technically) valid even though it actually is partially moved-out. Callers can subsequently access that moved-out result via things like `future.get()`, `future.result()`, `future.value()`, etc. - these access an already-moved-out result which is/can be surprising.
* Reasons `Future::get() &&` is better: its semantics are more obvious and user-testable. It moves-out the Future, leaving it with `future.valid() == false`.

Reviewed By: yfeldblum

Differential Revision: D8704033

fbshipit-source-id: 246f35c6320425e4b76ad11882bf55169988b772

folly/executors/test/ExecutorTest.cpp

@@ -284,5 +284,5 @@ TEST(Executor, DoNothingExecutor) {
   x.add({});
 
   EXPECT_TRUE(f.isReady());
-  EXPECT_THROW(f.get(), folly::BrokenPromise);
+  EXPECT_THROW(std::move(f).get(), folly::BrokenPromise);
 }

folly/experimental/test/FutureDAGTest.cpp

@@ -271,5 +271,5 @@ TEST_F(FutureDAGTest, DestroyBeforeComplete) {
     f = localDag->go();
   }
   barrier->wait();
-  ASSERT_NO_THROW(f.get());
+  ASSERT_NO_THROW(std::move(f).get());
 }

folly/fibers/test/FibersTest.cpp

@@ -1620,7 +1620,7 @@ void singleBatchDispatch(ExecutorT& executor, int batchSize, int index) {
 
   auto indexCopy = index;
   auto result = batchDispatcher.add(std::move(indexCopy));
-  EXPECT_EQ(folly::to<std::string>(index), result.get());
+  EXPECT_EQ(folly::to<std::string>(index), std::move(result).get());
 }
 
 TEST(FiberManager, batchDispatchTest) {
@@ -1720,7 +1720,7 @@ void doubleBatchOuterDispatch(
 
   auto indexCopy = index;
   auto result = batchDispatcher.add(std::move(indexCopy));
-  EXPECT_EQ(folly::to<std::string>(index), result.get());
+  EXPECT_EQ(folly::to<std::string>(index), std::move(result).get());
 }
 
 TEST(FiberManager, doubleBatchDispatchTest) {