[Ada] fix crash on Secondary_Stack_Size with discriminant

[Pierre-Marie de Rodat via Gcc-patches]

This patch fixes a crash caused by specifying the Secondary_Stack_Size
aspect of a task type as the value of a discriminant of the task type,
and then declaring a record component whose type is the task type,
constrained to a discriminant of the record.

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

gcc/ada/

        * sem_ch3.adb (Get_Discr_Value): Copy the result. Otherwise, the
        "tree" can be malformed (a DAG, not a tree) because
        Get_Discr_Value could be returning some subtree already attached
        to the tree.
diff --git a/gcc/ada/sem_ch3.adb b/gcc/ada/sem_ch3.adb
--- a/gcc/ada/sem_ch3.adb
+++ b/gcc/ada/sem_ch3.adb
@@ -13963,7 +13963,7 @@ package body Sem_Ch3 is
               or else D = CR_Discriminant (Discr_Id)
               or else Corresponding_Discriminant (D) = Discr_Id
             then
-               return Node (E);
+               return New_Copy_Tree (Node (E));
             end if;
 
             Next_Discriminant (D);
@@ -13987,7 +13987,7 @@ package body Sem_Ch3 is
             E := First_Elmt (Constraints);
             while Present (D) loop
                if D = Discr_Id then
-                  return Node (E);
+                  return New_Copy_Tree (Node (E));
                end if;
 
                Next_Discriminant (D);