[llvm] [Windows][test] Fix "LLVM" test failures when LLVM_WINDOWS_PREFER_FORWARD_SLASH is ON (PR #184556)

[Hans Wennborg via llvm-commits]

https://github.com/zmodem commented:

Not sure about the llvm-cov tests, but the others look good to me.

https://github.com/llvm/llvm-project/pull/184556