Making sure normal enums and enum classes are distinct

Making sure normal enums and enum classes are distinct

Summary:
With enum classes reaching candidate phase, we decided that switching on them
will be up to the developper (by using an explicit `EnumMember` wrapper class).
With the upcoming "wrapper less" syntax, switch on an enum class will be like switching on an instance, not an enum.
To achieve the same behavior, developers will have to explicitly use the `EnumMember` class to wrapper their own.

Because `enum` and `enum classes` have different semantic, I want to make sure we never confuse one for the other in the typechecker.
This diff makes sure `enum class E` won't be considered as a normal `enum`

Reviewed By: francesco-zappa-nardelli

Differential Revision: D25755050

fbshipit-source-id: 178dd8bf614cdc4daae8c6c873fd53e674f2919b