Fix compilation issues on Apple platforms. (#1641)

Summary:
Pull Request resolved: https://github.com/facebook/folly/pull/1641

There's a bug in C++ SDK shipped on Apple platforms that marks `std::optional`'s `value()` as unavailable. To resolve that we should use `operator*` instead.

Reviewed By: tcw165

Differential Revision: D30755147

fbshipit-source-id: 4edcf60a7d29abe7eaddceb66501535031553678

folly/experimental/coro/AsyncPipe.h

@@ -96,7 +96,7 @@ class AsyncPipe {
     if (onClosed != nullptr) {
       cancellationSource.emplace();
       onClosedCallback = std::make_unique<OnClosedCallback>(
-          cancellationSource.value(), std::move(onClosed));
+          *cancellationSource, std::move(onClosed));
     }
     auto guard =
         folly::makeGuard([cancellationSource = std::move(cancellationSource)] {