[ms-cxxabi] Mangle variadic template parameter packs

Unlike Itanium, there is no code to indicate the beginning of a
parameter pack.  I tested this with MSVC 2013, which is the only version
that implements variadic templates so far.

This is needed to compile APInt.cpp for the MS C++ ABI.

Reviewers: timurrrr

Differential Revision: http://llvm-reviews.chandlerc.com/D1077