[Ada] Improve debugging of task type discriminants

[Pierre-Marie de Rodat]

This patch improves the debugging information generated by the compiler
for task discriminant.

No test available.

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

2017-12-05  Javier Miranda  <mira...@adacore.com>

        * exp_ch9.adb (Install_Private_Data_Declarations): Add missing
        Debug_Info_Needed decoration of internally generated discriminal
        renaming declaration.


Index: exp_ch9.adb
===================================================================
--- exp_ch9.adb (revision 255412)
+++ exp_ch9.adb (working copy)
@@ -13450,6 +13450,12 @@
                        Selector_Name => Make_Identifier (Loc, Chars (D))));
                Add (Decl);
 
+               --  Set debug info needed on this renaming declaration even
+               --  though it does not come from source, so that the debugger
+               --  will get the right information for these generated names.
+
+               Set_Debug_Info_Needed (Discriminal (D));
+
                Next_Discriminant (D);
             end loop;
          end;