[Ada] Fix strange behavior of Object_Size for packed array subtype

[Ada] Fix strange behavior of Object_Size for packed array subtype

2018-05-31  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* gcc-interface/decl.c (gnat_to_gnu_entity) <discrete_type>: Do not
	deal with the ___XP suffix for packed array types here...
	<E_Array_Subtype>: ...or here when processing the implementation type
	but when processing the original type instead.  Do not reuse the DECL
	of the implementation type for the original type.  Tidy up.

From-SVN: r260999