Driver: Don't use DOS style paths on UNIX

We'd add, as a fallback, DOS style paths when using the driver using a
win32 triple.  On a UNIX-like platform, this isn't particularly helpful.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200507 91177308-0d34-0410-b5e6-96231b3b80d8