ada: Switch from E_Void to Is_Not_Self_Hidden

ada: Switch from E_Void to Is_Not_Self_Hidden

We had previously used Ekind = E_Void to indicate that a declaration is
self-hidden. We now use the Is_Not_Self_Hidden flag instead. This allows
us to avoid many "vanishing fields", which are (possibly-latent) bugs,
and we now enable the assertions in Atree that detect such bugs.

gcc/ada/

	* atree.adb (Check_Vanishing_Fields): Fix bug in the "blah type
	only" cases. Remove the special cases for E_Void. Misc cleanup.
	(Mutate_Nkind): Disallow mutating to the same kind.
	(Mutate_Ekind): Disallow mutating to E_Void.
	(From E_Void is still OK -- entities start out as E_Void by
	default.) Fix bug in statistics gathering -- was setting the wrong
	count. Enable Check_Vanishing_Fields for entities.
	* sem_ch8.adb (Is_Self_Hidden): New function.
	(Find_Direct_Name): Call Is_Self_Hidden to use the new
	Is_Not_Self_Hidden flag to determine whether a declaration is
	hidden from all visibility by itself. This replaces the old method
	of checking E_Void.
	(Find_Expanded_Name): Likewise.
	(Find_Selected_Component): Likewise.
	* sem_util.adb (Enter_Name): Remove setting of Ekind to E_Void.
	* sem_ch3.adb: Set the Is_Not_Self_Hidden flag in appropriate
	places. Comment fixes.
	(Inherit_Component): Remove setting of Ekind to E_Void.
	* sem_ch9.adb
	(Analyze_Protected_Type_Declaration): Update comment. Skip Itypes,
	which should not be turned into components.
	* atree.ads (Mutate_Nkind): Document error case.
	(Mutate_Ekind): Remove comments apologizing for E_Void mutations.
	Document error cases.