Allow 'co_return task' as way of implementing tail-recursion of tasks about cppcoro HOT 3 OPEN

Note that until we have access to the coroutine_handle<> await_suspend(coroutine_handle<> h) overload that guarantees tail-recursive resumption of another coroutine after suspending the current coroutine, the recursive invocation of nested coroutines may still consume some space on the regular stack, even if the resources used by coroutine frames are cleaned up eagerly.

See @GorNishanov's https://github.com/GorNishanov/clang/tree/tailcall branch for experimental implementation of await_suspend() overload in Clang.

See also https://github.com/lewissbaker/cppcoro/tree/lazy_task_tailcall branch for an example implementation of this co_return task feature for lazy_task

from cppcoro.

We won't be able to implement this for lazy_task<void> yet, however, since the current coroutines TS disallows mixing return_void() and return_value() in the same promise_type.

from cppcoro.

I've written up a draft paper that suggests relaxing the restrictions on co_return here:
https://github.com/lewissbaker/papers/blob/master/isocpp/relax-coreturn-restrictions.md

from cppcoro.