[Ada] Spurious accessibility error on allocator in generic instance

[Ada] Spurious accessibility error on allocator in generic instance

gcc/ada/

	* exp_ch4.adb (Expand_N_Type_Conversion): Remove flawed test for
	whether "statically deeper" accessibility rules apply to a given
	target type and instead use the new routine
	Statically_Deeper_Relation_Applies.
	(Statically_Deeper_Relation_Applies): Created to centralize the
	calculation of whether a target type within a conversion must
	have static accessibility checks.
	* sem_ch13.adb (Check_One_Function): Minor comment revision.