Let all Promise<T> be friend of Task

Summary:
[Folly][coro] Let all `Promise<T>` be friend of `Task`.

The code would not compile:

```lang=c++
coro::Task<void> taskVoid() {
  co_await task42();
  co_return;
}
```

```
folly/experimental/coro/Promise.h:84:28: error: 'viaInline' is a private member of 'folly::coro::Task<int>'
    return std::move(task).viaInline(executor_);
                           ^
folly/experimental/coro/tests/CoroTest.cpp:40:3: note: in instantiation of function template specialization 'folly::coro::Promise<void>::await_transform<int>' requested here
  co_await task42();
  ^
folly/experimental/coro/Task.h:60:13: note: declared private here
  Future<T> viaInline(folly::Executor* executor) && {
            ^
```

Reviewed By: andriigrynenko

Differential Revision: D7178238

fbshipit-source-id: 7dca6834ac56f4c9bdb4d702996b51e932f2aae6

folly/experimental/coro/Task.h

@@ -53,7 +53,8 @@ class Task {
   }
 
  private:
-  friend promise_type;
+  template <typename U>
+  friend class Promise;
 
   Future<T> viaInline(folly::Executor* executor) && {
     promise_->executor_ = executor;

folly/experimental/coro/tests/CoroTest.cpp

@@ -36,6 +36,22 @@ TEST(Coro, Basic) {
   EXPECT_EQ(42, future.get());
 }
 
+coro::Task<void> taskVoid() {
+  co_await task42();
+  co_return;
+}
+
+TEST(Coro, Basic2) {
+  ManualExecutor executor;
+  auto future = taskVoid().via(&executor);
+
+  EXPECT_FALSE(future.await_ready());
+
+  executor.drive();
+
+  EXPECT_TRUE(future.await_ready());
+}
+
 coro::Task<void> taskSleep() {
   co_await futures::sleep(std::chrono::seconds{1});
   co_return;