Revert "[lit] Use os.devnull instead of named temp files"

Revert "[lit] Use os.devnull instead of named temp files"

This reverts commit r272290. It breaks a test that depends on being able
to seek the /dev/null equivalent on Windows:

http://bb.pgr.jp/builders/ninja-clang-x64-mingw64-RA/builds/11360

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@272293 91177308-0d34-0410-b5e6-96231b3b80d8