[Ada] AI12-0226 Make objects more consistent

[Ada] AI12-0226 Make objects more consistent

2020-06-08  Arnaud Charlet  <charlet@adacore.com>

gcc/ada/

	* sem_ch8.adb (Analyze_Object_Renaming): Simplify code by moving
	many special cases to Is_Object_Reference and removing others by
	only checking renamings coming from sources.
	* sem_util.adb (Is_Object_Reference): Update for AI12-0226 and
	add more regular handling of 'Priority. Remove special cases no
	longer needed now that we are only checking renamings coming
	from sources.