c++: Reject std::initializer_list with bases [PR125647]

The following testcase shows that declaring std::initializer_list<T>
with bases can lead to ICEs, right now we were just testing that
it is a class template, not a union, and contains one pointer member
and one size_t member.

2026-08-06  Jakub Jelinek  <jakub@redhat.com>

	PR c++/125647
	* class.cc (finish_struct): Reject std::initializer_list template
	with bases.

	* g++.dg/cpp0x/initlist134.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>