Move from Unstable to Preview mode
Summary:
Moving Enum class label to Preview mode. Rollback strategy would be:
- all files that use the new syntax have the `enum_atom` unstable feature attribute, we can easily track and delete them
- remove all content that refers to `HH\Label`, which might be in file without the attribute, but can only be called from an entry point that has the attribute
Reviewed By: jamesjwu
Differential Revision:
D27904676
fbshipit-source-id:
5e88fa7cac9bf1747b17d806c21b70d78e17ed93