[flow] Use RString for reason of hint for string literal under autoco…

…mplete

Summary:
It shouldn't matter here, since this is never visible to the user. However, we still want to get rid of RCustom, so let's just use the description that we are also using below.

Changelog: [interna]

Reviewed By: gkz

Differential Revision: D56493014

fbshipit-source-id: 49c89e7b25379a472263df64281f67a3b8004f68

src/typing/statement.ml

@@ -672,7 +672,7 @@ module Make
   let string_literal_value cx ~as_const loc value =
     if Type_inference_hooks_js.dispatch_literal_hook cx loc then
       let (_, lazy_hint) = Type_env.get_hint cx loc in
-      let hint = lazy_hint (mk_reason (RCustom "literal") loc) in
+      let hint = lazy_hint (mk_reason RString loc) in
       let error () = EmptyT.at loc in
       Type_hint.with_hint_result hint ~ok:Base.Fn.id ~error
     else if as_const then