as MC is the only assembler we support.
This splits MS/Windows and GNU/Windows ASM infos into two seperate classes.
While there is currently only one difference, full MS C++ ABI support will
require many more.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@145409 91177308-0d34-0410-b5e6-96231b3b80d8