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

[Takuto Ikuta via llvm-commits]

atetubou wrote:

I made https://github.com/llvm/llvm-project/pull/193671 to fix `check-llvm`, can I take over the fix in my PR?

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