Possibly fix MSVC compilation after r256054.

I don't have any way to test MSVC compilation, but maybe this will fix
the error:

llvm/Support/TrailingObjects.h(286) : error C3210: 'TrailingObjectsBase' : access declaration can only be applied to a base class member
llvm/Support/TrailingObjects.h(337) : see reference to class template instantiation 'llvm::TrailingObjects<BaseTy,TrailingTys...>' being compiled
llvm/Support/TrailingObjects.h(286) : error C2602: 'llvm::trailing_objects_internal::TrailingObjectsBase::OverloadToken' is not a member of a base class of 'llvm::TrailingObjects<BaseTy,TrailingTys...>'
llvm/Support/TrailingObjects.h(91) : see declaration of 'llvm::trailing_objects_internal::TrailingObjectsBase::OverloadToken'

llvm-svn: 256068