Langkit_Support: remove the last anonymous access return types

Some versions of GNAT are known to complain about these and anyway they
are almost never correct: they are supposed to return an allocation
whose lifetime is bound to the caller's whereas we generally return
allocations whose lifetime is not statically known.

Just replace these anonymous access types with named access types,
creating some if needed. Behavior should not change.

For GitHub issue #235

langkit/support/langkit_support-adalog-eq_same.ads

@@ -132,7 +132,7 @@ package Langkit_Support.Adalog.Eq_Same is
          Eq_Data   : Equals_Data;
          Sloc_Info : String_Access := null) return Relation
       is
-        (Relation (Impl.Equals (L, R, Data, Eq_Data, Sloc_Info)));
+        (Impl.Equals (L, R, Data, Eq_Data, Sloc_Info));
 
    end Raw_Custom_Bind;
 

langkit/support/langkit_support-adalog-unify.ads

@@ -140,7 +140,7 @@ package Langkit_Support.Adalog.Unify is
       R_Data    : Right_C_Data := No_R_Data;
       Eq_Data   : Equals_Data := No_Equals_Data;
       Sloc_Info : String_Access := null)
-      return access Base_Relation'Class
+      return Relation
    is
      (Unify_Left.Create (L, R, R_Data, Eq_Data, Sloc_Info))
       with Inline;
@@ -151,7 +151,7 @@ package Langkit_Support.Adalog.Unify is
       L_Data    : Left_C_Data := No_L_Data;
       Eq_Data   : Equals_Data := No_Equals_Data;
       Sloc_Info : String_Access := null)
-      return access Base_Relation'Class
+      return Relation
    is
      (Unify_Right.Create (R, L, L_Data, Eq_Data, Sloc_Info))
       with Inline;

langkit/support/langkit_support-bump_ptr.adb

@@ -159,8 +159,6 @@ package body Langkit_Support.Bump_Ptr is
 
    package body Tagged_Alloc is
 
-      type Element_Access is access all Element_T;
-
       T : aliased Element_T;
 
       package Gen_Alloc is new
@@ -169,7 +167,7 @@ package body Langkit_Support.Bump_Ptr is
       function Dirty_Conv is new
          Ada.Unchecked_Conversion (Element_Access, Address_Access);
 
-      function Alloc (Pool : Bump_Ptr_Pool) return access Element_T is
+      function Alloc (Pool : Bump_Ptr_Pool) return Element_Access is
          --  This bit of code is actually quite funny. It is born out of the
          --  conflation of several unfortunate design choices in Ada:
 

langkit/support/langkit_support-bump_ptr.ads

@@ -94,8 +94,9 @@ package Langkit_Support.Bump_Ptr is
 
    generic
       type Element_T is tagged private;
+      type Element_Access is access all Element_T;
    package Tagged_Alloc is
-      function Alloc (Pool : Bump_Ptr_Pool) return access Element_T
+      function Alloc (Pool : Bump_Ptr_Pool) return Element_Access
          with Inline;
    end Tagged_Alloc;
    --  This generic allocation package can be used to allocate an object of