coroutines: Bind label_decl of original function to actor function

coroutines: Bind label_decl of original function to actor function

gcc/cp
    * coroutines.cc (transform_await_wrapper): Set actor funcion as
    new context of label_decl.
    (build_actor_fn): Fill new field of await_xform_data.

gcc/testsuite
    * g++.dg/coroutines/co-await-04-control-flow.C: Add label.