[Ada] Crash on aspect/pragma Linked_Section with -gnatR2

[Ada] Crash on aspect/pragma Linked_Section with -gnatR2

This patch modifies the output of the representation information related to
aspect or pragma Linker_Section, achieved with compiler switch -gnatR2. The
value of the section argument is now properly retrieved. Previously it was
assumed that the value is always a N_String_Literal, however the semantics
of the annotation allow for any static string expression, including a
reference to a static string.

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

--  linker_sections.ads

package Linker_Sections is
   LS_1 : constant String := "1";
   LS_2 : constant String := "2" & "2";
   LS_3 : constant String := LS_1 & "3";
   LS_4 : constant String := "4" & LS_2;

   Val_1  : Integer with Linker_Section => LS_1;
   Val_2  : Integer with Linker_Section => LS_2;
   Val_3  : Integer with Linker_Section => LS_3;
   Val_4  : Integer with Linker_Section => LS_4;
   Val_5  : Integer with Linker_Section => LS_1 & "5";
   Val_6  : Integer with Linker_Section => LS_2 & "6";
   Val_7  : Integer with Linker_Section => LS_3 & "7";
   Val_8  : Integer with Linker_Section => LS_4 & "8";
   Val_9  : Integer with Linker_Section => "9"  & LS_1;
   Val_10 : Integer with Linker_Section => "10" & LS_2;
   Val_11 : Integer with Linker_Section => "11" & LS_3;
   Val_12 : Integer with Linker_Section => "12" & LS_4;

   Val_13 : Integer; pragma Linker_Section (Val_13, LS_1);
   Val_14 : Integer; pragma Linker_Section (Val_14, LS_2);
   Val_15 : Integer; pragma Linker_Section (Val_15, LS_3);
   Val_16 : Integer; pragma Linker_Section (Val_16, LS_4);
   Val_17 : Integer; pragma Linker_Section (Val_17, LS_1 & "5");
   Val_18 : Integer; pragma Linker_Section (Val_18, LS_2 & "6");
   Val_19 : Integer; pragma Linker_Section (Val_19, LS_3 & "7");
   Val_20 : Integer; pragma Linker_Section (Val_20, LS_4 & "8");
   Val_21 : Integer; pragma Linker_Section (Val_21, "9"  & LS_1);
   Val_22 : Integer; pragma Linker_Section (Val_22, "10" & LS_2);
   Val_23 : Integer; pragma Linker_Section (Val_23, "11" & LS_3);
   Val_24 : Integer; pragma Linker_Section (Val_24, "12" & LS_4);
end Linker_Sections;

-----------------
-- Compilation --
-----------------

$ gcc -c -gnatR2s linker_sections.ads

2018-05-28  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* repinfo.adb (Expr_Value_S): New routine.
	(List_Linker_Section): Properly extract the value of the section
	argument.

From-SVN: r260825