Use promise in coroutine frame in actor function.

Use promise in coroutine frame in actor function.

By standard, coroutine body should be encapsulated in try-catch block
as following:
  try {
    // coroutine body
  } catch(...) {
    promise.unhandled_exception();
  }
Given above try-catch block is implemented in the coroutine actor
function called by coroutine ramp function, so the promise should
be accessed via actor function's coroutine frame pointer argument,
rather than the ramp function's coroutine frame variable.

This patch cleans code a bit to make fix easy.

gcc/cp
    * coroutines.cc (act_des_fn): New.
    (morph_fn_to_coro): Call act_des_fn to build actor/destroy decls.
    Access promise via actor function's frame pointer argument.
    (build_actor_fn, build_destroy_fn): Use frame pointer argument.