c++: Add self reference to define_aggregate created aggregates [PR123984]

c++: Add self reference to define_aggregate created aggregates [PR123984]

We weren't adding the DECL_SELF_REFERENCE_P TYPE_DECL to TYPE_FIELDS of
eval_define_aggregate created aggregates, which resulted in ICE in
accessible_base_p which relies on DECL_SELF_REFERENCE_P TYPE_DECL to be
present in base classes of other classes.

2026-02-09  Jakub Jelinek  <jakub@redhat.com>

	PR c++/123984
	* reflect.cc (eval_define_aggregate): Set TYPE_BEING_DEFINED on type
	after pushclass, call build_self_reference, remove assertion that
	TYPE_FIELDS (type) is NULL and instead set fields to
	TYPE_FIELDS (type).

	* g++.dg/reflect/define_aggregate6.C: New test.