[Ada] Crash on use of Loop_Entry, Result, and Old as actuals

[Ada] Crash on use of Loop_Entry, Result, and Old as actuals

2019-12-12  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* exp_ch6.adb (Expand_Call_Helper): Added null case for
	'Loop_Entry, 'Old, and 'Result when calculating whether to
	create extra accessibility parameters.
	* sem_util.adb (Dynamic_Accessibility_Level): Added null case
	for 'Loop_Entry, 'Old, and 'Result when  calculating
	accessibility level based on access-valued attributes.  Also
	added special handling for uses of 'Loop_Entry when used in its
	indexed component form.

From-SVN: r279280