ada: Mark attribute Initialized as ghost code

ada: Mark attribute Initialized as ghost code

Implement the SPARK RM change that defines attribute Initialized
as being ghost, i.e. only allowed where a ghost entity would be allowed.

gcc/ada/

	* ghost.adb (Check_Ghost_Context): Allow absence of Ghost_Id
	for attribute. Update error message to mention Ghost_Predicate.
	(Is_Ghost_Attribute_Reference): New query.
	* ghost.ads (Is_Ghost_Attribute_Reference): New query.
	* sem_attr.adb (Resolve_Attribute): Check ghost context for ghost
	attributes.