Since coroutine-ness is discovered lazily, we encounter the diagnostics
during each keyword parse.  We were not handling the case where a user code
failed to include fundamental information (e.g. the traits) in a graceful
manner.

Once we've emitted an error for this level of fail, then we suppress
additional copies (otherwise the same thing will be reported for every
coroutine keyword seen).

gcc/cp/ChangeLog:

2020-02-03  Iain Sandoe  <iain@sandoe.co.uk>

	* coroutines.cc (struct coroutine_info): Add a bool flag to note
	that we emitted an error for a bad function return type.
	(get_coroutine_info): Tolerate an unset info table in case of
	missing traits.
	(find_coro_traits_template_decl): In case of error or if we didn't
	find a type template, note we emitted the error and suppress
	duplicates.
	(find_coro_handle_template_decl): Likewise.
	(instantiate_coro_traits): Only check for error_mark_node in the
	return from lookup_qualified_name.
	(coro_promise_type_found_p): Reorder initialization so that we check
	for the traits and their usability before allocation of the info
	table.  Check for a suitable return type and emit a diagnostic for
	here instead of relying on the lookup machinery.  This allows the
	error to have a better location, and means we can suppress multiple
	copies.
	(coro_function_valid_p): Re-check for a valid promise (and thus the
	traits) before proceeding.  Tolerate missing info as a fatal error.

gcc/testsuite/ChangeLog:

2020-02-03  Iain Sandoe  <iain@sandoe.co.uk>

	* g++.dg/coroutines/pr93458-1-missing-traits.C: New test.
	* g++.dg/coroutines/pr93458-2-bad-traits.C: New test.
	* g++.dg/coroutines/pr93458-3-missing-handle.C: New test.
	* g++.dg/coroutines/pr93458-4-bad-coro-handle.C: New test.
	* g++.dg/coroutines/pr93458-5-bad-coro-type.C: New test.