[Ada] Fix wrong access check with access-to-unconstrained-array

The current implementation may create dangling references from a superset
of the alias set of the dummy pointer-to-array type when it exists.

gcc/ada/

	* gcc-interface/decl.cc (gnat_to_gnu_entity) <E_Array_Type>: Save
	and restore the alias set of the dummy pointer-to-array type.