driver: Map closed standard file descriptors to /dev/null

Utilize Process::FixupStandardFileDescriptors, introduced in r219170, to
guard against files from being treated as one of the standard file
descriptors.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219174 91177308-0d34-0410-b5e6-96231b3b80d8