This mangles entry points wmain, WinMain, wWinMain or DllMain as C
functions, to match the ABI for these functions.
We already did the same for these functions in MSVC mode, but we also
should do the same in the Itanium ABI.
This fixes PR38124.
Differential Revision: https://reviews.llvm.org/D49354
llvm-svn: 337146