[Ada] Use actual types instead of formal types consistently in debug info

This makes sure that the objects present in instantiations always have the
actual type instead of a local variant of the formal type in the debugging
information generated by the compiler (this was already the case when the
actual type is a record, a protected or a task type).

gcc/ada/

	* gcc-interface/decl.cc (Gigi_Cloned_Subtype): New function.
	(gnat_to_gnu_entity) <E_Signed_Integer_Subtype>: Call it to get the
	cloned subtype, if any.
	<E_Floating_Point_Subtype>: Likewise.
	<E_Array_Subtype>: Likewise.
	<E_Record_Subtype>: Likewise.
	<E_Access_Subtype>: Likewise.
	Deal with all cloned subtypes on the main path.