[Ada] Incorrect accessibility checks on functions calls

[Ada] Incorrect accessibility checks on functions calls

2020-06-10  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* exp_ch3.adb (Expand_N_Object_Declaration): Add condition to
	handle processing of objects initialized by a call to a function
	return an anonymous access type.
	* exp_ch6.adb, exp_ch6.ads
	(Has_Unconstrained_Access_Discriminants): Moved to sem_util.adb
	(Needs_Result_Accessibility_Level): Moved to sem_util.adb
	* sem_util.adb, sem_util.ads
	(Has_Unconstrained_Access_Discriminants): Moved from exp_ch6.adb
	(Needs_Result_Accessibility_Level): Moved from exp_ch6.adb
	* sem_res.adb (Valid_Conversion): Add condition for the special
	case where the operand of a conversion is the result of an
	anonymous access type