[Ada] Get rid of more references to Universal_Integer in expanded code

[Ada] Get rid of more references to Universal_Integer in expanded code

2020-06-02  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_aggr.adb (Others_Check): In the positional case, use the
	general expression for the comparison only when needed.
	* exp_attr.adb (Expand_Fpt_Attribute;): Use a simple conversion
	to the target type instead of an unchecked conversion to the
	base type to do the range check, as in the other cases.
	(Expand_N_Attribute_Reference) <Attribute_Storage_Size>: Do the
	Max operation in the type of the storage size variable, and use
	Convert_To as in the other cases.
	* tbuild.adb (Convert_To): Do not get rid of an intermediate
	conversion to Universal_Integer here...
	* sem_res.adb  (Simplify_Type_Conversion): ...but here instead.