c++: limit instantiation with ill-formed class [PR96286]

c++: limit instantiation with ill-formed class [PR96286]

I noticed that after the static_assert failures in lwg3466.cc, we got
various follow-on errors because we went ahead and tried to instantiate the
promise<T> member functions even after instantiating the class itself ran
into problems.  Interrupting instantiation of the class itself seems likely
to cause error-recovery problems, but preventing instantiation of member
functions seems strictly better for error-recovery.

This doesn't fix any of the specific testcases in PR96286, but addresses
part of that problem space.

	PR c++/96286

gcc/cp/ChangeLog:

	* cp-tree.h (struct lang_type): Add erroneous bit-field.
	(CLASSTYPE_ERRONEOUS): New.
	* pt.c (limit_bad_template_recursion): Check it.
	(instantiate_class_template_1): Set it.

libstdc++-v3/ChangeLog:

	* testsuite/30_threads/promise/requirements/lwg3466.cc:
	Remove dg-prune-outputs.

gcc/testsuite/ChangeLog:

	* g++.dg/template/access2.C: Split struct A.