[llvm] [lit] Use raw strings for backslash escapes to fix SyntaxWarnings (PR #70907)

[Thomas Preud'homme via llvm-commits]

https://github.com/RoboTux approved this pull request.

LGTM. Regarding who should be code owner I'm not sure, generally I would say it should be whoever is also owner for the tests as the file's content has more to do with the tests than lit in terms of why it's there. Since it might affect NVPTX tests, maybe some NVPTX backend owner.

https://github.com/llvm/llvm-project/pull/70907