[Ada] Unnesting bugs with array renamings generated for quantified expr

[Ada] Unnesting bugs with array renamings generated for quantified expr

2020-06-02  Gary Dismukes  <dismukes@adacore.com>

gcc/ada/

	* exp_unst.adb (Visit_Node): When visiting array attribute
	nodes, apply Get_Referenced_Object to the attribute prefix, to
	handle prefixes denoting renamed objects by picking up the Etype
	of the renamed object rather than the possibly unconstrained
	nominal subtype of the renaming declaration's Entity.
	* sem_util.ads (Get_Referenced_Object): Update comment to
	clearly indicate that any kind of node can be passed to this
	function.
	* sem_util.adb (Get_Referenced_Object): Add test of Is_Object to
	the condition, to allow for passing names that denote types and
	subtypes.