[Ada] Incorrect diagnostic on illegal with clause

This patch updates the detection of illegal with clauses which import private
child packages to properly detect a grandchild of Ada.

------------
-- Source --
------------

--  fake.ads

package Fake is
end Fake;

--  fake-ada.ads

package Fake.Ada is
end Fake.Ada;

--  fake-ada-text_io.ads

package Fake.Ada.Text_IO is
end Fake.Ada.Text_IO;

--  fake-ada-text_io-float_io.ads

private generic
   type Num is digits <>;

package Fake.Ada.Text_IO.Float_IO is
end Fake.Ada.Text_IO.Float_IO;

--  fake-float_io.ads

private generic
   type Num is digits <>;

package Fake.Float_IO is
end Fake.Float_IO;

--  main.ads

with Fake.Ada.Text_IO.Float_IO;
with Fake.Float_IO;

package Main is
end Main;

----------------------------
-- Compilation and output --
----------------------------

$ gcc -c main.ads
main.ads:1:06: unit in with clause is private child unit
main.ads:1:06: current unit must also have parent "Text_IO"
main.ads:2:06: unit in with clause is private child unit
main.ads:2:06: current unit must also have parent "Fake"

2018-01-11  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* sem_ch10.adb (Check_Private_Child_Unit): Ensure that the enclosing
	scope of package Ada is Standard.

From-SVN: r256515

gcc/ada/ChangeLog

+2018-01-11  Hristian Kirtchev  <kirtchev@adacore.com>
+
+	* sem_ch10.adb (Check_Private_Child_Unit): Ensure that the enclosing
+	scope of package Ada is Standard.
+
 2018-01-11  Ed Schonberg  <schonberg@adacore.com>
 
 	* sem_ch13.adb (Replace_Type_Ref): Handle properly reference to a

gcc/ada/sem_ch10.adb

@@ -6,7 +6,7 @@
 --                                                                          --
 --                                 B o d y                                  --
 --                                                                          --
---          Copyright (C) 1992-2017, Free Software Foundation, Inc.         --
+--          Copyright (C) 1992-2018, Free Software Foundation, Inc.         --
 --                                                                          --
 -- GNAT is free software;  you can  redistribute it  and/or modify it under --
 -- terms of the  GNU General Public License as published  by the Free Soft- --
@@ -2995,6 +2995,8 @@ package body Sem_Ch10 is
                   if Ekind (Priv_Child) = E_Generic_Package
                     and then Chars (Priv_Child) in Text_IO_Package_Name
                     and then Chars (Scope (Scope (Priv_Child))) = Name_Ada
+                    and then Scope (Scope (Scope (Priv_Child))) =
+                               Standard_Standard
                   then
                      Error_Msg_NE
                        ("& is a nested package, not a compilation unit",