Decide on approach for handling memory allocation failure of coroutine frame

[lewissbaker]

The default approach is for the call to the coroutine function to throw an exception itself if allocation of the coroutine frame fails (typically due to std::bad_alloc).

For coroutine types that return a task, should we instead implement the promise_type::get_return_object_on_allocation_failure method to return a special task value that defers throwing the std::bad_alloc exception until the returned task is awaited?

This would make it possible to declare coroutine functions as noexcept (provided captured parameter copies all have noexcept copy/move constructors).