ada: Make class-wide Max_Size_In_Storage_Elements return a large value

Max_Size_In_Storage_Elements is supposed to return a value greater or
equal to what is passed for any heap allocation for an object of the
type. For a tagged type T, we don't know the allocation size for
descendants; therefore T'Class'Max_Size_In_Storage_Elements should
return a huge number. In particular, it now returns Storage_Count'Last,
which is greater than any possible heap allocation.

Previously, T'Class'Max_Size_In_Storage_Elements was returning
the same value as T'Max_Size_In_Storage_Elements, which was
wrong.

gcc/ada/ChangeLog:

	* exp_attr.adb (Attribute_Max_Size_In_Storage_Elements):
	Return Storage_Count'Last converted to universal_integer.