[Ada] Enforce deferred constant completion rules

[Ada] Enforce deferred constant completion rules

If a constrained subtype is given when a deferred constant is declared,
then the subtype given in the completion is required (at compile time)
to be subject to a statically matching constraint. This rule was not
properly enforced in some cases and constructs that should have been
rejected were incorrectly accepted.

gcc/ada/

	* sem_ch3.adb (Check_Possible_Deferred_Completion): Delete
	Prev_Obj_Def formal parameter.  Reorganize code so that
	statically matching check is also performed in the case where
	the subtype given in the initial declaration is constrained and
	the subtype given in the completion is not.