Revert "Revert "[lit] Generalized /dev/null support on Windows.""

Summary:
This reverts commit r328596.

Checking if the arguments are strings before testing if they contain "/dev/null".

Reviewers: rnk

Reviewed By: rnk

Subscribers: delcypher, llvm-commits

Differential Revision: https://reviews.llvm.org/D44914

llvm-svn: 328603