Adapt conversion from old parsetree

Now that function attributes and first-argument attributes are split, they need to be re-joined in the old parse tree.
This means that after a ppx, both will be on the first argument.

Author: cristianoc

compiler/ml/ast_mapper_to0.ml

@@ -101,7 +101,9 @@ module T = struct
     | Ptyp_arrow {arg; ret; arity} -> (
       let lbl = Asttypes.to_noloc arg.lbl in
       let typ0 =
-        arrow ~loc ~attrs lbl (sub.typ sub arg.typ) (sub.typ sub ret)
+        arrow ~loc
+          ~attrs:(attrs @ sub.attributes sub arg.attrs)
+          lbl (sub.typ sub arg.typ) (sub.typ sub ret)
       in
       match arity with
       | None -> typ0

tests/syntax_tests/data/ast-mapping/expected/FunWithAttribute.res.txt

@@ -1 +1 @@
-type fn = (~foo: string) => int
+type fn = (@as("something") ~foo: string) => int