[Ada] Extend legality of Scalar_Storage_Order to formal types

[Ada] Extend legality of Scalar_Storage_Order to formal types

This patch extends the legality of the GNAT attribute Scalar_Storage_Order,
to apply to formal private types. Previously this extension applied only
in GNAT_Mode, to support instantiations of Ada.Sequential_IO, but it is more
generally useful.

The following must compile quietly:

----
with Memory_View_Generic;
procedure Main is
   type T is array (1..10) of integer;
   package OK is new Memory_View_Generic (T);

   type T2 is new Long_Float;
   package Wrong is new Memory_View_Generic (T2);
begin
   null;
end;
----
with System;
generic
   type Source_Type is private;
package Memory_View_Generic is
   -- various declarations ...
   SSO : System.Bit_Order := Source_Type'Scalar_Storage_Order;
end Memory_View_Generic;

2018-05-21  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_attr.adb (Analyze_Attribute, case Scalar_Storage_Order): The
	attribute reference is legal within a generic unit when the prefix is a
	formal private type.

From-SVN: r260444