ada: Disable a couple of static accessibility checks in dynamic cases

In accordance with the RM 3.10.2(19,19.1,19.2) rules.

gcc/ada/ChangeLog:

	* exp_ch4.adb (Expand_Allocator_Expression): In the case of access
	discriminants, pass Zero_On_Dynamic_Level to compute the static
	accessibility level of the expression.
	* sem_res.adb (Check_Aliased_Parameter): Pass Zero_On_Dynamic_Level
	to compute the static accessibility level of actual parameters.