Workaround MSVC bug when using TrailingObjects from a template.

MSVC appears to be getting confused as to whether OverloadToken is
supposed to be public or not.

This was discovered by code in Swift, and has been reported to
microsoft by hughbe:
https://connect.microsoft.com/VisualStudio/feedback/details/3116517

Differential Revision: https://reviews.llvm.org/D29880

llvm-svn: 296497