lld-link: Emit warning if one each of {main,wmain} and {WinMain,wWinMain}...

lld-link: Emit warning if one each of {main,wmain} and {WinMain,wWinMain} exist and no /subsystem: flag is passed.

Similar to link.exe's LNK4031.
https://reviews.llvm.org/D51076

llvm-svn: 340420