[-cxx-abi microsoft] Mangle declarations inside extern "C"

Summary:
This is a first step to getting extern "C" working properly inside
clang.  There are a number of quirks but mangling declarations inside
such a function are a good first step.

Reviewers: timurrrr, pcc, cdavis5x

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1655