Handle type deduction of auto and decltype(auto) with reference expression

Handle type deduction of auto and decltype(auto) with reference expression

gcc/cp
    * coroutines.cc (build_co_await): Call convert_from_reference
    to wrap co_await_expr with indirect_ref which avoid
    reference/non-reference type confusion.

    (co_await_expander):  Sink to call_expr if await_resume
    is wrapped by indirect_ref.

gcc/testsuite
    * g++.dg/coroutines/co-await-14-return-ref-to-auto.C: New test.