Fix misfiring unused-local-typedef under clang

Summary:
[Folly] Fix misfiring `unused-local-typedef` under clang.

The problem is fixed in most recent versions of clang, but appears with some previous versions.

```
folly/experimental/coro/Task.h:318:11: error: unused type alias 'handle_t' [-Werror,-Wunused-local-typedef]
    using handle_t =
          ^
```

Reviewed By: andriigrynenko

Differential Revision: D13569416

fbshipit-source-id: 94ae07455c2d08a1516c10baf1e3a16f2a29225f

folly/experimental/coro/Task.h

@@ -323,10 +323,9 @@ class FOLLY_NODISCARD Task {
 
 template <typename T>
 auto detail::TaskPromiseBase::await_transform(Task<T>&& t) noexcept {
-  class Awaiter {
-    using handle_t =
-        std::experimental::coroutine_handle<detail::TaskPromise<T>>;
+  using handle_t = std::experimental::coroutine_handle<detail::TaskPromise<T>>;
 
+  class Awaiter {
    public:
     explicit Awaiter(handle_t coro) noexcept : coro_(coro) {}