In some cases where a function result type has an access discriminant
part, Ada requires that the execution of a return statement include a
check that the access discriminant does not designate an object whose
accessibility level is too deep (Ada RM 6.5(21)). This check was being
incorrectly omitted in some cases where the discriminant value
designates a not-explicitly-aliased parameter of the function (or some
part thereof).  Correct this omission.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_ch6.adb (Check_Return_Construct_Accessibility): Modify
	generation of accessibility checks to be more consolidated and
	get triggered properly in required cases.
	* sem_util.adb (Accessibility_Level): Add extra check within
	condition to handle aliased formals properly in more cases.