Generator local scope resources are not cleaned up after coroutine object going out of scope

I would have expected the local scope resources within a generator coroutine to be cleaned up at coroutine object destruction, but it didn't.

```cpp
cobalt::generator gg()
{
boost::scope::defer_guard release = [&] { std::println("exiting gg"); };

co_yield 10;
co_yield 11;
co_return 12;
}

cobalt::task t12()
{
{
auto gen = gg();
co_await gen;
} // <- `gen` out of scope here, `release` should be destructed but it doesn't.
std::println("gen is out of scope");
}

cobalt::main co_main(int, char **)
{
{
co_await t12();
}
std::println("t12 is out of scope");
co_return 0;
}

```

Expected order:

```
exiting gg
gen is out of scope
t12 is out of scope
```

Actual order:

```
gen is out of scope
t12 is out of scope
exiting gg
```

This is very surprising (considering that `co_await`-ing does have proper cancellation support that cleans up the local scope objects) and it has caused a nasty bug that took me hours to diagnose.