c++: inter-cluster import order [PR 99283]

c++: inter-cluster import order [PR 99283]

I finally managed to reduce the testcase without hitting other bugs.
This problem is caused by discovering a duplicate in the middle of
reading in the entity in question.  I had thougt the import seeding at
the beginning of a cluster prevented that, but it is insufficient.
Specifically an earlier cluster in the same module can cause the
import of a duplicate.  Although clusters within a module are
well-ordered, there is no ordering between clusters of one module and
clusters of another module.  And thus we can get duplicate declaration
loops.  This prevents the problem by also seeding references to
earlier clusters in the same module.  As the FIXME notes, it is
sufficient to reference a single entity in any particular earlier
cluster, plus, we also could determine the implicit dependencies and
prune that seeding even further.  I do not do that -- it decrease the
loading that will happen, but would reduce the serialization size.  As
ever, let's get correctness first.

	PR c++/99283
	gcc/cp/
	* module.cc (trees_out::decl_node): Adjust importedness reference
	assert.
	(module_state::intercluster_seed): New.  Seed both imports and
	inter-cluster references.  Broken out of ...
	(module_state::write_cluster): ... here.  Call it.
	gcc/testsuite/
	* g++.dg/modules/pr99283-6.h: New.
	* g++.dg/modules/pr99283-6_a.H: New.
	* g++.dg/modules/pr99283-6_b.H: New.
	* g++.dg/modules/pr99283-6_c.C: New.
	* g++.dg/modules/hdr-init-1_c.C: Adjust scan.
	* g++.dg/modules/indirect-3_c.C: Adjust scan.
	* g++.dg/modules/indirect-4_c.C: Adjust scan.
	* g++.dg/modules/lambda-3_b.C: Adjust scan.
	* g++.dg/modules/late-ret-3_c.C: Adjust scan.
	* g++.dg/modules/pr99425-1_b.H: Adjust scan.
	* g++.dg/modules/pr99425-1_c.C: Adjust scan.