Fix false warning messages about missing return in coroutine.

Fix false warning messages about missing return in coroutine.

The patch sets current_function_returns_value flag in templates for
all co_return/co_yield/co_await cases, as well as for ramp function.

gcc/cp/ChangeLog
    * coroutines.cc (finish_co_await_expr): Set return value flag.
    (finish_co_yield_expr, morph_fn_to_coro): Ditto.

gcc/testsuite/ChangeLog
    * g++.dg/coroutines/co-return-warning-1.C: New test.