c++/reflection: add assert to eval_type_trait

c++/reflection: add assert to eval_type_trait

As discussed in
<https://gcc.gnu.org/pipermail/gcc-patches/2026-January/705756.html>.

Since we check eval_is_type in process_metafunction:

  if (eval_is_type (ht) != boolean_true_node)
     return throw_exception_nontype (loc, ctx, fun, non_constant_p,
				     jump_target);

finish_trait_expr should never return error_mark_node.  We can ensure
that it's so by adding an assert.

gcc/cp/ChangeLog:

	* reflect.cc (eval_type_trait): Assert that finish_trait_expr didn't
	return error_mark_node.

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