Summary:
Code may pass a callback which captures an object with a destructor which mutates through a stored reference, triggering heap-use-after-free or stack-use-after-scope.

```lang=c++
void performDataRace() {
  auto number = std::make_unique<int>(0);
  auto guard = folly::makeGuard([&number] { *number = 1; });
  folly::via(getSomeExecutor(), [guard = std::move(guard)]() mutable {}).wait();
  // data race - we may wake and destruct number before guard is destructed on the
  // executor thread, which is both stack-use-after-scope and heap-use-after-free!
}
```

We can avoid this condition by always destructing the provided functor before setting any result on the promise.

Retry at {D4982969}.

Reviewed By: andriigrynenko

Differential Revision: D5058750

fbshipit-source-id: 4d1d878b4889e5e6474941187f03de5fa84d3061