[Ada] Elaboration order v4.0 and linker switches

[Pierre-Marie de Rodat]

This patch adds a missing functionality with respect to elaboration
order v3.0.  Units carry an attribute called Elab_Position which among
other things controls the sorting of linker switches by gnatbind.
Setting the proper position ensures the gnatbind will output the linker
switches in an order compatible with what gnatlink expects.

No simple test available as this requires a Windows cross-compiler.

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-07-10  Hristian Kirtchev  <kirtc...@adacore.com>

gcc/ada/

        * bindo-elaborators.adb (Elaborate_Units): Set attribute
        Elab_Position of all elaborated units.
        (Set_Unit_Elaboration_Positions): New routine.
--- gcc/ada/bindo-elaborators.adb
+++ gcc/ada/bindo-elaborators.adb
@@ -274,6 +274,10 @@ package body Bindo.Elaborators is
       --  Determine whether vertex Vertex of library graph G is suitable for
       --  weak elaboration.
 
+      procedure Set_Unit_Elaboration_Positions (Order : Unit_Id_Table);
+      pragma Inline (Set_Unit_Elaboration_Positions);
+      --  Set the ALI.Units positions of all elaboration units in order Order
+
       procedure Trace_Component
         (G    : Library_Graph;
          Comp : Component_Id;
@@ -750,6 +754,11 @@ package body Bindo.Elaborators is
          if Status = Order_OK then
             Validate_Elaboration_Order (Order);
 
+            --  Set attribute Elab_Position of table ALI.Units for all units in
+            --  the elaboration order.
+
+            Set_Unit_Elaboration_Positions (Order);
+
             --  Output the dependencies among units when switch -e (output
             --  complete list of elaboration order dependencies) is active.
 
@@ -1304,6 +1313,23 @@ package body Bindo.Elaborators is
              and then Is_Weakly_Elaborable_Vertex (G, Vertex);
       end Is_Suitable_Weakly_Elaborable_Vertex;
 
+      ------------------------------------
+      -- Set_Unit_Elaboration_Positions --
+      ------------------------------------
+
+      procedure Set_Unit_Elaboration_Positions (Order : Unit_Id_Table) is
+         U_Id : Unit_Id;
+
+      begin
+         for Position in Unit_Id_Tables.First ..
+                         Unit_Id_Tables.Last (Order)
+         loop
+            U_Id := Order.Table (Position);
+
+            ALI.Units.Table (U_Id).Elab_Position := Position;
+         end loop;
+      end Set_Unit_Elaboration_Positions;
+
       ---------------------
       -- Trace_Component --
       ---------------------