[ADT] Removed VariadicFunction

Summary:
It is not used. It uses macro-based unrolling instead of variadic
templates, so it is not idiomatic anymore, and therefore it is a
questionable API to keep "just in case".

Subscribers: mgorny, dmgreen, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66961

llvm-svn: 370441