[Ada] Implement AI12-0032 fixes for 'Old attribute accessibility

[Ada] Implement AI12-0032 fixes for 'Old attribute accessibility

2020-06-18  Steve Baird  <baird@adacore.com>

gcc/ada/

	* sem_util.adb (Object_Access_Level): Treat a 'Old attribute
	reference like an aggregate in determining its static
	accessibility level; after the evaluation of the relevant
	post-condition, the implicitly declared constant associated with
	an Old attribute reference ceases to exist. Similarly for
	Loop_Entry attribute.
	* exp_ch6.adb (Expand_Call_Helper): For an attribute reference
	that is expanded into a reference to an implicitly declared
	constant (e.g., a 'Old or 'Loop_Entry attribute), compute the
	dynamic accessibility level of that constant by looking at the
	declaration of the constant (as opposed to looking at the
	attribute reference).