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

[James Henderson via llvm-commits]

https://github.com/jh7370 commented:

Thinking more about the llvm-objdump and llvm-cov tests: shouldn't these tests show that the correct slash is used, based on the value of `LLVM_WINDOWS_PREFER_FORWARD_SLASH`?

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