Disable the GNUstep v2 ABI on Windows.

The code remains so that we can potentially reenable it in a point
release, but the driver will reject it.  Several issues were raised
during testing that made it clear that this was not quite ready for
general consumption.

Approved by: Hans Wennborg

git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_70@341354 91177308-0d34-0410-b5e6-96231b3b80d8