[coro] Fix co_await of void type.

[coro] Fix co_await of void type.

gcc/cp
2020-01-21  Iain Sandoe  <iain@sandoe.co.uk>
    Bin Cheng  <bin.cheng@linux.alibaba.com>

* coroutines.cc (coro_promise_type_found_p): Check for NULL return
from complete_type_or_else.
(register_param_uses): Likewise.
(build_co_await): Do not try to use complete_type_or_else for void
types, otherwise for incomplete types, check for NULL return from
complete_type_or_else.

gcc/testsuite
2020-01-21  Bin Cheng  <bin.linux@linux.alibaba.com>

* g++.dg/coroutines/co-await-void_type.C: New test.