[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:

Re: the regressions on the lit tests. 

It turned out a bit tricky because lit tests run lit itself with forward slash version of %t etc. I'm trying to find a reasonable workaround to make those tests pass. 

Re: PR title, description

I was able to modify the PR title now.  I also added the stats before/after this patch. PTAL

Re: CI for `DLLVM_WINDOWS_PREFER_FORWARD_SLASH=ON`

Ok. We will check [the document for buildbot](https://llvm.org/docs/HowToAddABuilder.html). 

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