c++/reflection: overzealous complete_type in consteval_only_p [PR125280]

This bug report shows that we generate a bogus "has incomplete type"
error for Element::mData in type12.C with -freflection: consteval_only_p
attempts to complete Array when finishing mData, but mData uses Element
which is currently being defined, so we can't complete it yet.  We could
fix this bogus error by checking can_complete_type_without_circularity
before calling complete_type in consteval_only_p, but I'm no longer so
sure that we must call complete_type at all.  This patch removes that
call.  One consequence of that is that we don't produce the "outside
a constant-evaluated context" error for C::mData (only when mData is
defined outside the class as with A::mData above).  This behavior
matches clang++ though so I'm not too worried about it.

type13.C shows that even without the complete_type we can still get
to consteval_only_p_walker::walk with a member with erroneous type
which currently crashes.

	PR c++/125280

gcc/cp/ChangeLog:

	* reflect.cc (consteval_only_p): Don't complete_type.
	(consteval_only_p_walker::walk): Return false for
	error_mark_node.

gcc/testsuite/ChangeLog:

	* g++.dg/reflect/init19.C: New test.
	* g++.dg/reflect/type12.C: New test.
	* g++.dg/reflect/type13.C: New test.

Reviewed-by: Patrick Palka <ppalka@redhat.com>