[Ada] Fix problem with run-time library units and SPARK mode

[Arnaud Charlet]

The test for library units was wrong, resulting in run-time
library units being incorrectly marked with SPARK_Mode on
when a configuration pragma file had pragma SPARK_Mode (ON).

The following program

     1. with Ada.Text_IO; use Ada.Text_IO;
     2. procedure SPragLib is
     3. begin
     4.    Put_Line ("Hello World!");
     5. end SPragLib;

compiled with -gnatdtf should yield a file that has only one
reference to the SPARK_Mode field, not dozens as before.

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

2014-01-24  Robert Dewar  <de...@adacore.com>

        * sem.adb (Sem): Avoid premature reference to Current_Sem_Unit
        (this was causing Is_Main_Unit_Or_Main_Unit_Spec to be set wrong,
        leading to wrong handling of SPARK_Mode for library units).


Index: sem.adb
===================================================================
--- sem.adb     (revision 207026)
+++ sem.adb     (working copy)
@@ -1312,18 +1312,19 @@
       S_Outer_Gen_Scope   : constant Entity_Id        := Outer_Generic_Scope;
       S_Style_Check       : constant Boolean          := Style_Check;
 
+      Curunit : constant Unit_Number_Type := Get_Cunit_Unit_Number (Comp_Unit);
+      --  New value of Current_Sem_Unit
+
       Generic_Main : constant Boolean :=
-                       Nkind (Unit (Cunit (Main_Unit)))
-                         in N_Generic_Declaration;
+        Nkind (Unit (Cunit (Main_Unit))) in N_Generic_Declaration;
       --  If the main unit is generic, every compiled unit, including its
       --  context, is compiled with expansion disabled.
 
       Is_Main_Unit_Or_Main_Unit_Spec : constant Boolean :=
-         Current_Sem_Unit = Main_Unit
+         Curunit = Main_Unit
            or else
              (Nkind (Unit (Cunit (Main_Unit))) = N_Package_Body
-               and then Library_Unit (Cunit (Main_Unit)) =
-                  Cunit (Current_Sem_Unit));
+               and then Library_Unit (Cunit (Main_Unit)) = Cunit (Curunit));
       --  Configuration flags have special settings when compiling a predefined
       --  file as a main unit. This applies to its spec as well.
 
@@ -1393,7 +1394,7 @@
       end if;
 
       Compiler_State   := Analyzing;
-      Current_Sem_Unit := Get_Cunit_Unit_Number (Comp_Unit);
+      Current_Sem_Unit := Curunit;
 
       --  Compile predefined units with GNAT_Mode set to True, to properly
       --  process the categorization stuff. However, do not set GNAT_Mode