This patch adds a flag Is_Past_Self_Hiding_Point. When False,
this will replace E_Void as the indicator for a premature use of
a declaration within itself -- for example, "X : T := X;".

One might think this flag should be called something like
Is_Hidden_From_All_Visibility, reversing the sense of
Is_Past_Self_Hiding_Point. We don't do that because we want
Is_Past_Self_Hiding_Point to be initially False by default (and we have
no mechanism for defaulting to True), and because it doesn't exactly
match the RM definition of "hidden from all visibility" (for
example, for record components).

This is work in progress; more changes are needed before we
can remove all Mutate_Ekind(..., E_Void).

gcc/ada/

	* einfo.ads (Is_Past_Self_Hiding_Point): Document.
	* gen_il-fields.ads (Is_Past_Self_Hiding_Point): Add to list of
	fields.
	* gen_il-gen-gen_entities.adb (Is_Past_Self_Hiding_Point): Declare
	in all entities.
	* exp_aggr.adb: Set Is_Past_Self_Hiding_Point as appropriate.
	* sem.adb: Likewise.
	* sem_aggr.adb: Likewise.
	* sem_ch11.adb: Likewise.
	* sem_ch12.adb: Likewise.
	* sem_ch5.adb: Likewise.
	* sem_ch7.adb: Likewise.
	* sem_prag.adb: Likewise.
	* sem_ch6.adb: Likewise.
	(Set_Formal_Mode): Minor cleanup: Move from spec.
	* sem_ch6.ads:
	(Set_Formal_Mode): Minor cleanup: Move to body.
	* cstand.adb: Call Set_Is_Past_Self_Hiding_Point on all entities
	as soon as they are created.
	* comperr.adb (Compiler_Abort): Minor cleanup -- use 'in' instead
	of 'or else'.
	* debug.adb: Minor comment cleanups.