[Ada] Small cleanup in handling of Ada 2012 implicit dereferences

[Ada] Small cleanup in handling of Ada 2012 implicit dereferences

2020-06-12  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* sem_ch4.adb (Try_Container_Indexing): Replace call to
	First_Discriminant by Get_Reference_Discriminant to get the
	reference discriminant.
	* sem_ch13.adb (Check_Indexing_Functions): Likewise.
	* sem_ch5.adb (Preanalyze_Range): Call Get_Reference_Discriminant
	to get the reference discriminant.
	* sem_util.adb (Is_OK_Variable_For_Out_Formal): Treat all
	Ada 2012 implicit dereferences in only one place.
	(Is_Variable): Minor tweak.