coroutines: Improve error recovery [PR94817, PR94829, PR95087].

coroutines: Improve error recovery [PR94817, PR94829, PR95087].

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].

Diagnosing bad uses of 'return' in coroutines is somewhat
tricky, since the user can use the keyword before we know
that the function is a coroutine (where such returns are not
permitted).  At present, we are just doing a check for any
use of 'return' and erroring on that.  However, we can't then
pass the function body on, since it will contain unlowered
coroutine trees.

This avoids the issue by dropping the entire function body
under that circumstance.

	Backport c7100843831147a034fe37d231c54ac53ceace45 and a1bb808504643e6c3c0df0fdd68a941ed2a64c7f0

gcc/cp/ChangeLog:

	PR c++/94817
	PR c++/94829
	PR c++/95087
	* 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.  If we see
	an early fatal error, drop the erroneous function body.
	* 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:

	PR c++/94817
	PR c++/94829
	PR c++/95087
	* 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.
	* g++.dg/coroutines/co-return-syntax-08-bad-return.C:
	Adjust the testcase to do the compile (rather than an
	-fsyntax-only parse).
	* g++.dg/coroutines/coro1-ret-int-yield-int.h
	(MISSING_INITIAL_SUSPEND, MISSING_FINAL_SUSPEND): New.