Do not use an access type discriminant for a mutable type

... This is not allowed.

To circumvent, define a local class wide access type.

For https://gitlab.adacore-it.com/eng/toolchain/gnat/-/issues/993

liblsp_base/source/lsp-json_streams.ads

@@ -29,9 +29,11 @@ with VSS.Text_Streams;
 package LSP.JSON_Streams is
 --   pragma Elaborate_Body;
 
+   type JSON_Reader is access all VSS.JSON.Pull_Readers.JSON_Pull_Reader'Class;
+
    type JSON_Stream
      (Is_Server_Side : Boolean := False;
-      R : access VSS.JSON.Pull_Readers.JSON_Pull_Reader'Class := null)
+      R              : JSON_Reader := null)
        is limited new Ada.Streams.Root_Stream_Type with private;
    --  Stream implemented over JSON document
    --
@@ -106,7 +108,7 @@ private
 
    type JSON_Stream
      (Is_Server_Side : Boolean := False;
-      R : access VSS.JSON.Pull_Readers.JSON_Pull_Reader'Class := null)
+      R              : JSON_Reader := null)
    is limited new Ada.Streams.Root_Stream_Type with
    record
       W : Write_Stream;

source/client/lsp-clients.adb

@@ -636,7 +636,7 @@ package body LSP.Clients is
          use all type VSS.JSON.Streams.JSON_Stream_Element_Kind;
 
          R  : aliased VSS.JSON.Pull_Readers.Simple.JSON_Simple_Pull_Reader;
-         JS : aliased LSP.JSON_Streams.JSON_Stream (False, R'Access);
+         JS : aliased LSP.JSON_Streams.JSON_Stream (False, R'Unchecked_Access);
 
       begin
          R.Set_Stream (Memory'Unchecked_Access);