Insert default return_void at the end of coroutine body

Insert default return_void at the end of coroutine body

Exception in coroutine is not correctly handled because the default
return_void call is now inserted before the finish suspend point,
rather than at the end of the original coroutine body.  This patch
fixes the issue by expanding code as following:
  co_await promise.initial_suspend();
  try {
    // The original coroutine body

    promise.return_void(); // The default return_void call.
  } catch (...) {
    promise.unhandled_exception();
  }
  final_suspend:
  // ...

gcc/cp/
    * coroutines.cc (build_actor_fn): Factor out code inserting the
    default return_void call to...
    (morph_fn_to_coro): ...here, also hoist local var declarations.

gcc/testsuite/
    * g++.dg/coroutines/torture/co-ret-15-default-return_void.C: New.