[Ada] Reuse Get_Pragma_Arg to handle pragma argument associations

[Pierre-Marie de Rodat via Gcc-patches]

Code cleanup related to looking at pragma Thread_Local_Storage.
Semantics is unaffected.

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

gcc/ada/

        * exp_ch3.adb (Build_Init_Statements): Reuse Get_Pragma_Arg.
        * exp_prag.adb (Arg_N): Likewise.
diff --git a/gcc/ada/exp_ch3.adb b/gcc/ada/exp_ch3.adb
--- a/gcc/ada/exp_ch3.adb
+++ b/gcc/ada/exp_ch3.adb
@@ -3319,11 +3319,9 @@ package body Exp_Ch3 is
                         --  Pragma case
 
                         if Nkind (Ritem) = N_Pragma then
-                           Exp := First (Pragma_Argument_Associations (Ritem));
-
-                           if Nkind (Exp) = N_Pragma_Argument_Association then
-                              Exp := Expression (Exp);
-                           end if;
+                           Exp :=
+                             Get_Pragma_Arg
+                               (First (Pragma_Argument_Associations (Ritem)));
 
                            --  Conversion for Priority expression
 


diff --git a/gcc/ada/exp_prag.adb b/gcc/ada/exp_prag.adb
--- a/gcc/ada/exp_prag.adb
+++ b/gcc/ada/exp_prag.adb
@@ -105,12 +105,10 @@ package body Exp_Prag is
          end if;
       end loop;
 
-      if Present (Arg)
-        and then Nkind (Arg) = N_Pragma_Argument_Association
-      then
-         return Expression (Arg);
+      if Present (Arg) then
+         return Get_Pragma_Arg (Arg);
       else
-         return Arg;
+         return Empty;
       end if;
    end Arg_N;