Since

  Clang: Provide highlighting for identifier under cursor
  commit ca72c294

the LocalUseMap, used for some refactoring actions, was not updated
anymore. Fall back to the built-in implementation for the LocalUseMap
because it contains pointers to built-in AST that clang obviously can't
provide.

Task-number: QTCREATORBUG-18607
Change-Id: I08762fe457835d4d83aca719febcb3497ee9696b
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>
Reviewed-by: Robert Loehning <robert.loehning@qt.io>
Reviewed-by: Marco Bubke <marco.bubke@qt.io>