coroutines: Improve error recovery [PR94817, PR94829].

coroutines: Improve error recovery [PR94817, PR94829].

When we have completely missing key information (e.g. the
coroutine_traits) or a partially transformed function body, we
need to try and balance returning useful information about
failures with the possibility that some part of the diagnostics
machinery or following code will not be able to handle the
state.

The PRs (and revised testcase) point to cases where that processing
has failed.

This revises the process to avoid special handling for the
ramp, and falls back on the same code used for regular function
fails.

There are test-cases (in addition to the ones for the PRs) that now
cover all early exit points [where the transforms are considered
to have failed in a manner that does not allow compilation to
continue].

gcc/cp/ChangeLog:

2020-05-07  Iain Sandoe  <iain@sandoe.co.uk>

	PR c++/94817
	PR c++/94829
	* coroutines.cc (morph_fn_to_coro): Set unformed outline
	functions to error_mark_node.  For early error returns suppress
	warnings about missing ramp return values.  Fix reinstatement
	of the function body on pre-existing initial error.
	* decl.c (finish_function): Use the normal error path for fails
	in the ramp function, do not try to compile the helpers if the
	transform fails.

gcc/testsuite/ChangeLog:

2020-05-07  Iain Sandoe  <iain@sandoe.co.uk>

	PR c++/94817
	PR c++/94829
	* g++.dg/coroutines/coro-missing-final-suspend.C: New test.
	* g++.dg/coroutines/coro-missing-initial-suspend.C: New test.
	* g++.dg/coroutines/coro-missing-promise-yield.C: Check for
	continuation of compilation.
	* g++.dg/coroutines/coro-missing-promise.C: Likewise.
	* g++.dg/coroutines/coro-missing-ret-value.C: Likewise
	* g++.dg/coroutines/coro-missing-ret-void.C: Likewise
	* g++.dg/coroutines/coro-missing-ueh-3.C: Likewise
	* g++.dg/coroutines/pr94817.C: New test.
	* g++.dg/coroutines/pr94829.C: New test.