Backport: Warning for unused record fields (ocaml/ocaml#10232)

Closes ocaml-flambda/flambda-backend#646.

Squashed commit of the following:

commit 1a2370e
Author: Leo White <leo@lpw25.net>
Date:   Mon Mar 22 14:39:26 2021 +0000

    Add Changes entry

commit 47a14b4
Author: Leo White <leo@lpw25.net>
Date:   Wed Feb 17 13:53:02 2021 +0000

    Add warning for unused labels

commit 3a128ca
Author: Leo White <leo@lpw25.net>
Date:   Wed Feb 17 13:52:42 2021 +0000

    Remove unused labels

commit f7ae40d
Author: Leo White <leo@lpw25.net>
Date:   Tue Feb 16 09:31:28 2021 +0000

    Add tests for unused labels

asmcomp/asmgen.ml

@@ -235,7 +235,6 @@ let end_gen_implementation ?toplevel ~ppf_dump
 
 type middle_end =
      backend:(module Backend_intf.S)
-  -> filename:string
   -> prefixname:string
   -> ppf_dump:Format.formatter
   -> Lambda.program
@@ -246,15 +245,15 @@ let asm_filename output_prefix =
     then output_prefix ^ ext_asm
     else Filename.temp_file "camlasm" ext_asm
 
-let compile_implementation ?toplevel ~backend ~filename ~prefixname ~middle_end
+let compile_implementation ?toplevel ~backend ~prefixname ~middle_end
       ~ppf_dump (program : Lambda.program) =
   compile_unit ~output_prefix:prefixname
     ~asm_filename:(asm_filename prefixname) ~keep_asm:!keep_asm_file
     ~obj_filename:(prefixname ^ ext_obj)
     (fun () ->
       Ident.Set.iter Compilenv.require_global program.required_globals;
       let clambda_with_constants =
-        middle_end ~backend ~filename ~prefixname ~ppf_dump program
+        middle_end ~backend ~prefixname ~ppf_dump program
       in
       end_gen_implementation ?toplevel ~ppf_dump clambda_with_constants)
 

asmcomp/asmgen.mli

@@ -18,7 +18,6 @@
 (** The type of converters from Lambda to Clambda. *)
 type middle_end =
      backend:(module Backend_intf.S)
-  -> filename:string
   -> prefixname:string
   -> ppf_dump:Format.formatter
   -> Lambda.program
@@ -28,7 +27,6 @@ type middle_end =
 val compile_implementation
    : ?toplevel:(string -> bool)
   -> backend:(module Backend_intf.S)
-  -> filename:string
   -> prefixname:string
   -> middle_end:middle_end
   -> ppf_dump:Format.formatter

asmcomp/asmpackager.ml

@@ -132,7 +132,6 @@ let make_package_object ~ppf_dump members targetobj targetname coercion
         program, Closure_middle_end.lambda_to_clambda
     in
     Asmgen.compile_implementation ~backend
-      ~filename:targetname
       ~prefixname
       ~middle_end
       ~ppf_dump