Expansion of attributes Input/Output for unchecked unions didn't work,
because it looked for a No_Elist where the AST has an empty list. Now we
look for a default value of a first discriminant (because the default
values must be given either for all or for none of the discrminants).

Also, it was wrong to insert an N_Raise_Program_Error as an action and
leave the original N_Attribute_Reference node, because Gigi doesn't know
how to expand input or output attribute references. Now we rewrite the
attribute reference just like a neighbouring code which raises
Program_Error with a reason of PE_Stream_Operation_Not_Allowed.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* exp_attr.adb (Expand_N_Attribute_Reference): Fix expansion of
	attributes Input and Output for unchecked unions.
	* sem_case.ads: Fix typo "disriminant" and refill comment.