[Ada] Qualify internal access-to-subprogram types as not null

[Pierre-Marie de Rodat]

When an access-to-subprogram type is qualified as not null, then the
compiler is likely to generate a more efficient code. This patch adds
such qualifiers to the internal runtime bodies, so no clients of those
runtime units are affected.

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

gcc/ada/

        * libgnat/g-alleve.adb (Bit_Operation): Now a not-null type.
        * libgnat/g-sechas.adb (Fill_Buffer_Access): Likewise.
        * libgnat/s-dwalin.adb (Callback): Likewise.
diff --git a/gcc/ada/libgnat/g-alleve.adb b/gcc/ada/libgnat/g-alleve.adb
--- a/gcc/ada/libgnat/g-alleve.adb
+++ b/gcc/ada/libgnat/g-alleve.adb
@@ -258,7 +258,7 @@ package body GNAT.Altivec.Low_Level_Vectors is
       pragma Convention (LL_Altivec, vspltisx);
 
       type Bit_Operation is
-        access function
+        not null access function
         (Value  : Component_Type;
          Amount : Natural) return Component_Type;
 


diff --git a/gcc/ada/libgnat/g-sechas.adb b/gcc/ada/libgnat/g-sechas.adb
--- a/gcc/ada/libgnat/g-sechas.adb
+++ b/gcc/ada/libgnat/g-sechas.adb
@@ -38,7 +38,7 @@ package body GNAT.Secure_Hashes is
                  "0123456789abcdef";
 
    type Fill_Buffer_Access is
-     access procedure
+     not null access procedure
        (M     : in out Message_State;
         SEA   : Stream_Element_Array;
         First : Stream_Element_Offset;


diff --git a/gcc/ada/libgnat/s-dwalin.adb b/gcc/ada/libgnat/s-dwalin.adb
--- a/gcc/ada/libgnat/s-dwalin.adb
+++ b/gcc/ada/libgnat/s-dwalin.adb
@@ -130,7 +130,7 @@ package body System.Dwarf_Lines is
       Code :        uint32) return String;
    --  Extract a file name from the prologue
 
-   type Callback is access procedure (C : in out Dwarf_Context);
+   type Callback is not null access procedure (C : in out Dwarf_Context);
    procedure For_Each_Row (C : in out Dwarf_Context; F : Callback);
    --  Traverse each .debug_line entry with a callback