[-cxx-abi microsoft] Mangle user defined entry points properly

Summary:
Functions named "main", "wmain", "WinMain", "wWinMain", and "DllMain"
are never mangled regardless of linkage, even when compiling for kernel
mode.
Depends on D1655

Reviewers: timurrrr, pcc, rnk, whunt

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1670