[Ada] Spurious error when using current instance of type

[Ada] Spurious error when using current instance of type

gcc/ada/

	* exp_ch3.adb (Build_Assignment): Replace current instance of
	type with Init_Proc formal.
	* sem_attr.adb (OK_Self_Reference): Handle recognition of
	Current_Instance to detect certain expansion.
	* sem_ch4.adb (Analyze_One_Call): Set actual's type when the
	actual in question is a current instance and its corresponding
	formal is an incomplete type.
	* sem_util.adb (Is_Current_Instance): Add check for incomplete
	views and add comment.