ada: Clean up vanishing entity fields

ada: Clean up vanishing entity fields

Fix all the failures caused by enabling Check_Vanishing_Fields on
entities in all cases except the case of converting to or from E_Void.
But leave Check_Vanishing_Fields disabled by default (controlled by
-gnatd_v flag), because it might be too slow even for assertions-on
mode, and we should deal with the E_Void cases eventually.

The failures are fixed either by adding calls to Reinit_Field_To_Zero,
or by changing which entities have which fields.

Note that in a series of Reinit_Field_To_Zero calls, the optional
Old_Ekind parameter is only useful on the first such call.

gcc/ada/

	* atree.adb
	(Check_Vanishing_Fields): Disable the check for "root/base type
	only" fields. This is a bug fix -- if we're checking some subtype
	S, we don't want to reach over to the root or base type and
	Reinit_Field_To_Zero of that, thus modifying the field for lots of
	subtypes other than S. Disable in the to/from E_Void cases. Misc
	cleanup.
	* gen_il-gen-gen_entities.adb: Define First_Entity, Last_Entity,
	and Stored_Constraint for all type entities, because there are too
	many cases where Reinit_Field_To_Zero would otherwise be needed.
	In any case, it seems cleaner to have First_Entity and Last_Entity
	defined in the same entity kinds.
	* einfo.ads:
	(First_Entity, Last_Entity, Stored_Constraint): Update comments to
	reflect gen_il-gen-gen_entities.adb changes.
	(Lit_Hash): Add missing "[root type only]" comment.
	* exp_ch5.adb: Add Reinit_Field_To_Zero calls for vanishing
	fields.
	* sem_ch10.adb: Likewise.
	* sem_ch6.adb: Likewise.
	* sem_ch7.adb: Likewise.
	* sem_ch8.adb: Likewise.
	* sem_ch3.adb: Likewise. Also remove now-unnecessary
	Reinit_Field_To_Zero calls.