coroutines: Prevent repeated error messages for missing promise.

coroutines: Prevent repeated error messages for missing promise.

If the user's coroutine return type omits the mandatory promise
type then we will currently restate that error each time we see
a coroutine keyword, which doesn't provide any new information.
This suppresses all but the first instance in each coroutine.

gcc/cp/ChangeLog:

2020-02-04  Iain Sandoe  <iain@sandoe.co.uk>

	* coroutines.cc (find_promise_type): Delete unused forward
	declaration.
	(struct coroutine_info): Add a bool for no promise type error.
	(coro_promise_type_found_p): Only emit the error for a missing
	promise once in each affected coroutine.

gcc/testsuite/ChangeLog:

2020-02-04  Iain Sandoe  <iain@sandoe.co.uk>

	* g++.dg/coroutines/coro-missing-promise.C: New test.