[flow] Allow private name rename

Summary:
It has worked before, but once we added `textDocument/prepareRename` support to resolve conflict between relay and us, it was broken, because we forgot to allow rename in private names. This diff fixes it.

Changelog: [ide] Support rename on private properties and methods.

Reviewed By: mariusschulz

Differential Revision: D77733751

fbshipit-source-id: 9e6ace76e7452de31493b781544c2c0fee99c75b

Author: SamChou19815

src/services/references/prepareRenameSearcher.ml

@@ -14,6 +14,10 @@ class searcher ~contains_loc =
     method! identifier ((loc, _) as id) =
       if contains_loc loc then raise (Found loc);
       id
+
+    method! private_name ((loc, _) as id) =
+      if contains_loc loc then raise (Found loc);
+      id
   end
 
 let search_rename_loc ast cursor_loc =