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

[Pierre-Marie de Rodat]

This aligns the handling of Ada 2012 implicit dereferences in the
Is_OK_Variable_For_Out_Formal predicate wih that in Is_Variable,
and removes unreachable code as a result.

This also makes more use of the Get_Reference_Discriminant helper
function to retrieve the reference discriminant of a type.

No functional changes.

Tested on x86_64-pc-linux-gnu, committed on trunk

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.
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