c++: Fix up extract_ref [PR125111]

c++: Fix up extract_ref [PR125111]

https://eel.is/c++draft/meta.reflection.extract#5.2 requests that
is_convertible_v<remove_reference_t<U>(*)[], remove_reference_t<T>(*)[]>
is tested and if it not true, the function throws.
If U/T are references to function/method/array[], we try to build
array of types which are not valid in arrays and ICE shortly after that.
The following patch makes sure we don't create arrays in that case
and instead just throw the exception.  We can't have references to void
or references to references, so I think the other problematic cases
of creating arrays can't trigger.

2026-05-07  Jakub Jelinek  <jakub@redhat.com>

	PR c++/125111
	* reflect.cc (extract_ref): Throw instead of trying to build
	arrays of functions, methods or arrays with NULL TYPE_DOMAIN.

	* g++.dg/reflect/extract12.C: New test.

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