Mark @as used early in constructor declarations, and turn on unused check.

A couple of test files were mis-using `@as`.

Author: cristianoc

jscomp/frontend/bs_ast_invariant.ml

@@ -28,8 +28,8 @@
 *)
 let is_bs_attribute txt =
   match txt with
-  (* TODO #6636: | "as "| "int" *)
-  | "bs" | "config" | "ignore" | "optional" | "string" | "uncurry" | "unwrap" ->
+  (* TODO #6636: "int" *)
+  | "as" | "bs" | "config" | "ignore" | "optional" | "string" | "uncurry" | "unwrap" ->
     true
   | _ -> false
 
@@ -113,6 +113,10 @@ let emit_external_warnings : iterator =
         super.label_declaration self lbl);
     constructor_declaration =
       (fun self ({pcd_name = {txt; loc}} as ctr) ->
+        let _ =
+          Ast_untagged_variants.process_tag_type
+            ctr.pcd_attributes (* mark @as used in variant cases *)
+        in
         (match txt with
         | "false" | "true" | "()" ->
           Location.raise_errorf ~loc "%s can not be redefined " txt

jscomp/test/AsInUncurriedExternals.res

@@ -12,5 +12,5 @@ let mo = makeOptions
 
 let options = mo(~name="foo", ())
 
-let shouldNotFail: (~objectMode: @as(json`false`) _, ~name: string) => int = (~objectMode, ~name) =>
+let shouldNotFail: (~objectMode: _, ~name: string) => int = (~objectMode, ~name) =>
   3

jscomp/test/polyvar_convert.res

@@ -1,5 +1,5 @@
 type t = [
-  | @as("x") #a
+  | #a
   | #b
 ]