[Ada] Implement AI12-0275 (Make subtype_mark optional in object renamings)

[Ada] Implement AI12-0275 (Make subtype_mark optional in object renamings)

2020-06-04  Gary Dismukes  <dismukes@adacore.com>

gcc/ada/

	* par-ch3.adb (P_Identifier_Declarations): Add parsing of object
	renamings that have neither a subtype_mark nor an
	access_definition. Issue an error if the version is earlier than
	Ada_2020, and suggest using -gnatX.
	* sem_ch8.adb (Analyze_Object_Renaming): Handle
	object_renaming_declarations that don't have an explicit
	subtype. Errors are issued when the name is inappropriate or
	ambiguous, and otherwise the Etype of the renaming entity is set
	from the Etype of the renamed object.
	* sem_util.adb (Has_Null_Exclusion): Allow for the case of no
	subtype given in an N_Object_Renaming_Declaration.
	* sprint.adb (Sprint_Node_Actual): Handle printing of
	N_Object_Renaming_Declarations that are specified without an
	explicit subtype.