Enable basic folly::Future support for XROS builds.

Summary:
`folly/io/async:async_base` is not fully migrated to XROS, mitigate futures implementation for now:
* Craft out `fibers::Baton` and `IO pool` dependencies from futures.
* Add futures to CI targets.
* Add build time test to ensure `folly::Future<Unit>` symbols are generated.

This closely follows mitigation in `AROS` branch:
https://www.internalfb.com/code/aros/[a0c19baba13b]/xros/third-party/folly/folly/futures/Future.h

(any suggestions on how to properly proceed with such change are greatly appreciated!)

Reviewed By: egorich239

Differential Revision: D31497625

fbshipit-source-id: c0b2406496ca3c4f69ddadd61dbd1d75f309e8a1

folly/futures/Future-inl.h

@@ -34,13 +34,21 @@
 #include <folly/futures/detail/Core.h>
 #include <folly/lang/Pretty.h>
 
+#if defined(FOLLY_DISABLE_FUTURE_FIBERS_BATON)
+#include <folly/synchronization/Baton.h>
+#endif
+
 namespace folly {
 
 class Timekeeper;
 
 namespace futures {
 namespace detail {
+#if defined(FOLLY_DISABLE_FUTURE_FIBERS_BATON)
+typedef folly::Baton<> FutureBatonType;
+#else
 typedef folly::fibers::Baton FutureBatonType;
+#endif
 } // namespace detail
 } // namespace futures
 
@@ -461,13 +469,18 @@ class WaitExecutor final : public folly::Executor {
 
   void drive() {
     baton_.wait();
+
+#if !defined(FOLLY_DISABLE_FUTURE_FIBERS_BATON)
     fibers::runInMainContext([&]() {
+#endif
       baton_.reset();
       auto funcs = std::move(queue_.wlock()->funcs);
       for (auto& func : funcs) {
         std::exchange(func, nullptr)();
       }
+#if !defined(FOLLY_DISABLE_FUTURE_FIBERS_BATON)
     });
+#endif
   }
 
   using Clock = std::chrono::steady_clock;
@@ -476,14 +489,18 @@ class WaitExecutor final : public folly::Executor {
     if (!baton_.try_wait_until(deadline)) {
       return false;
     }
+#if !defined(FOLLY_DISABLE_FUTURE_FIBERS_BATON)
     return fibers::runInMainContext([&]() {
+#endif
       baton_.reset();
       auto funcs = std::move(queue_.wlock()->funcs);
       for (auto& func : funcs) {
         std::exchange(func, nullptr)();
       }
       return true;
+#if !defined(FOLLY_DISABLE_FUTURE_FIBERS_BATON)
     });
+#endif
   }
 
   void detach() {

folly/futures/Future.h

@@ -33,7 +33,9 @@
 #include <folly/executors/DrivableExecutor.h>
 #include <folly/executors/TimedDrivableExecutor.h>
 #include <folly/experimental/coro/Traits.h>
+#if !defined(FOLLY_DISABLE_FUTURE_FIBERS_BATON)
 #include <folly/fibers/Baton.h>
+#endif
 #include <folly/functional/Invoke.h>
 #include <folly/futures/Portability.h>
 #include <folly/futures/Promise.h>
@@ -2144,8 +2146,10 @@ template <class F>
 auto when(bool p, F&& thunk)
     -> decltype(std::declval<invoke_result_t<F>>().unit());
 
+#if !defined(FOLLY_DISABLE_FUTURE_FIBERS_BATON)
 SemiFuture<Unit> wait(std::unique_ptr<fibers::Baton> baton);
 SemiFuture<Unit> wait(std::shared_ptr<fibers::Baton> baton);
+#endif
 
 /**
  * Returns a lazy SemiFuture constructed by f, which also ensures that ensure is