Don't try and instantiate an invalid function in DiscriminatedPtrDetail

Don't try and instantiate an invalid function in DiscriminatedPtrDetail

Summary:
THe issue is quite simple: Regardless of the control flow, the previous implementation of this was instantiating `ApplyVisitor1<V,R>` which declares a return type, but never returns.
This refactors `ApplyVisitor1` and `ApplyConstVisitor1` so that the part of the control flow that was previously never reached simply doesn't exist anymore.

Reviewed By: yfeldblum

Differential Revision: D3479584

fbshipit-source-id: 605a48e39bba6dc14df1af1e76b55ea60f3e69d5