[ASTMatchers] Add own version of VariadicFunction.

Summary:
llvm::VariadicFunction is only being used by ASTMatchers.
Having our own copy here allows us to remove the other one from llvm/ADT.
Also, we can extend the API to meet our needs without modifying the common
implementation.

Reviewers: alexfh

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D18275

llvm-svn: 264417