fix rescript-lang#4542, keep the compatibility with rls

Author: bobzhang

jscomp/ext/bsc_args.ml

@@ -41,6 +41,7 @@
     | Unit_clear of bool ref 
 
  type spec =
+   | Unit_dummy  
    | Unit of unit_action
    | String of string_action 
 
@@ -128,6 +129,7 @@ let parse_exn  ~usage ~argv ?(start=1) ?(finish=Array.length argv) (speclist : t
       match Ext_spec.assoc3 speclist s with 
       | Some action -> begin       
           begin match action with 
+            | Unit_dummy -> ()
             | Unit r -> 
               begin match r with 
                 | Unit_set r -> r := true

jscomp/ext/bsc_args.mli

@@ -34,7 +34,12 @@ type unit_action =
   | Unit_lazy of unit lazy_t
   | Unit_set of bool ref
   | Unit_clear of bool ref
-type spec = Unit of unit_action | String of string_action
+
+
+type spec = 
+  | Unit_dummy  
+  | Unit of unit_action   
+  | String of string_action
 
 
 type t = (string * spec * string) array

jscomp/main/js_main.ml

@@ -502,6 +502,8 @@ let buckle_script_flags : (string * Bsc_args.spec * string) array =
     "-warn-help", unit_call Warnings.help_warnings, 
     "Show description of warning numbers";
 
+    "-c", Unit_dummy,
+    "*internal* keep the compatibility with RLS"
   |]