Remove VariadicOperatorMatcherInterface as it is redundant with logic from DynTypedMatcher.

Summary:
The generic variadic matcher is faster (one less virtual function call
per match) and doesn't require template instantiations which reduces
compile time and binary size.
Registry.cpp.o generates ~14% less symbols and compiles ~7.5% faster.
The change also speeds up our clang-tidy benchmark by ~2%.

Reviewers: klimek

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D6278