[Ada] Refine type from Nat to Pos in Make_Index

[Pierre-Marie de Rodat]

Refine types to make the code easier to understand. Semantics is not
affected; all the callers have been already respecting the more
restrictive type of parameter anyway.

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

gcc/ada/

        * sem_ch3.ads, sem_ch3.adb (Make_Index): Refined type of
        parameter.
        (Constrain_Index): Likewise.
        (Array_Type_Declaration): Refine type of a local counter
        variable; remove a trivially useless initialization.
diff --git a/gcc/ada/sem_ch3.adb b/gcc/ada/sem_ch3.adb
--- a/gcc/ada/sem_ch3.adb
+++ b/gcc/ada/sem_ch3.adb
@@ -440,7 +440,7 @@ package body Sem_Ch3 is
       Related_Nod  : Node_Id;
       Related_Id   : Entity_Id;
       Suffix       : Character;
-      Suffix_Index : Nat);
+      Suffix_Index : Pos);
    --  Process an index constraint S in a constrained array declaration. The
    --  constraint can be a subtype name, or a range with or without an explicit
    --  subtype mark. The index is the corresponding index of the unconstrained
@@ -5987,9 +5987,9 @@ package body Sem_Ch3 is
       Element_Type  : Entity_Id;
       Implicit_Base : Entity_Id;
       Index         : Node_Id;
-      Nb_Index      : Nat;
+      Nb_Index      : Pos;
       Priv          : Entity_Id;
-      Related_Id    : Entity_Id := Empty;
+      Related_Id    : Entity_Id;
 
    begin
       if Nkind (Def) = N_Constrained_Array_Definition then
@@ -14100,7 +14100,7 @@ package body Sem_Ch3 is
       Related_Nod  : Node_Id;
       Related_Id   : Entity_Id;
       Suffix       : Character;
-      Suffix_Index : Nat)
+      Suffix_Index : Pos)
    is
       Def_Id : Entity_Id;
       R      : Node_Id := Empty;
@@ -19120,7 +19120,7 @@ package body Sem_Ch3 is
      (N            : Node_Id;
       Related_Nod  : Node_Id;
       Related_Id   : Entity_Id := Empty;
-      Suffix_Index : Nat       := 1)
+      Suffix_Index : Pos       := 1)
    is
       R      : Node_Id;
       T      : Entity_Id;


diff --git a/gcc/ada/sem_ch3.ads b/gcc/ada/sem_ch3.ads
--- a/gcc/ada/sem_ch3.ads
+++ b/gcc/ada/sem_ch3.ads
@@ -195,7 +195,7 @@ package Sem_Ch3 is
      (N            : Node_Id;
       Related_Nod  : Node_Id;
       Related_Id   : Entity_Id := Empty;
-      Suffix_Index : Nat       := 1);
+      Suffix_Index : Pos       := 1);
    --  Process an index that is given in an array declaration, an entry
    --  family declaration or a loop iteration. The index is given by an index
    --  declaration (a 'box'), or by a discrete range. The later can be the name