Checkpoints: rework use of last checkpoint format version when writing

gnatcov always use the version of the checkpoint file format when
writing a SID/checkpoint. Using this fact, move the "Version"
information from read/write common code to read-specific code.

Author: pmderodat

tools/gnatcov/checkpoints.adb

@@ -443,8 +443,7 @@ package body Checkpoints is
       Context  : access Coverage.Context;
       Purpose  : Checkpoint_Purpose)
    is
-      Version : constant Checkpoint_Version := Checkpoint_Version'Last;
-      SF      : Ada.Streams.Stream_IO.File_Type;
+      SF : Ada.Streams.Stream_IO.File_Type;
    begin
       Create (SF, Out_File, Filename);
 
@@ -455,12 +454,11 @@ package body Checkpoints is
            (Root_Stream_Type with
             Stream   => Stream (SF),
             Filename => To_Unbounded_String (Filename),
-            Version  => Version,
             Purpose  => Purpose);
          Supported_Levels : Levels_Type := Current_Levels;
       begin
          Checkpoint_Header'Write
-           (CSS.Stream, (Version => Version, others => <>));
+           (CSS.Stream, (Version => Checkpoint_Version'Last, others => <>));
 
          Checkpoint_Purpose'Write (CSS.Stream, Purpose);
 

tools/gnatcov/checkpoints.ads

@@ -241,9 +241,6 @@ package Checkpoints is
          Filename : Unbounded_String;
          --  Name of the checkpoint being written/read
 
-         Version : Checkpoint_Version;
-         --  Format version for the checkpoint being written/read
-
          Purpose : Checkpoint_Purpose;
          --  Purpose for the checkpoint being written/read
       end record;
@@ -257,14 +254,6 @@ package Checkpoints is
       Item   : Stream_Element_Array);
    --  Read/write from/to underlying stream
 
-   use type Interfaces.Unsigned_32;
-   function Version_Less
-     (SS : Stateful_Stream'Class; Than : Checkpoint_Version) return Boolean
-   is (SS.Version < Than)
-     with Inline;
-   --  This is provided as a function to prevent the compiler from generating
-   --  "can never be greater than" warnings.
-
    function Purpose_Of
      (SS : Stateful_Stream'Class) return Checkpoint_Purpose
    is (SS.Purpose)
@@ -279,9 +268,20 @@ package Checkpoints is
    --  Global state shared across phases of a checkpoint load
 
    type Checkpoint_Load_State is new Stateful_Stream with record
+      Version : Checkpoint_Version;
+      --  Format version for the checkpoint being read
+
       Relocations : Checkpoint_Relocations;
    end record;
 
+   use type Interfaces.Unsigned_32;
+   function Version_Less
+     (CLS : Checkpoint_Load_State; Than : Checkpoint_Version) return Boolean
+   is (CLS.Version < Than)
+     with Inline;
+   --  This is provided as a function to prevent the compiler from generating
+   --  "can never be greater than" warnings.
+
    procedure Checkpoint_Save
      (Filename : String;
       Context  : access Coverage.Context;