[lld] [llvm] [LIT] Use forward slashes in substitutions when LLVM_WINDOWS_PREFER_FORWARD_SLASH is set (PR #179865)

[Junji Watanabe via llvm-commits]

Jwata wrote:

For the lit test regressions:

I had to relax the path matching because the lit test itself runs `lit.py` to check the outputs which has backslashes, while the substitutions used in the test expectations still use forward  slashes... Please let me know if anyone has a better approach. Given that only two tests are affected, I hope this is acceptable.

For CI checks with `-DLLVM_WINDOWS_PREFER_FORWARD_SLASH=ON`:

@matts1 already proposed https://github.com/llvm/llvm-zorg/pull/746 to add it to clang-x64-windows-msvc. 
But, it's not a blocker for this patch to be submitted. We will figure out how to catch regressions.

So, please take another look on this. Thank you

https://github.com/llvm/llvm-project/pull/179865