[clang] [llvm] [LLVM][Clang][Cygwin] Fix building Clang for Cygwin (PR #134494)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

> Okay, you were right. Keeping this in a single description hurts readability a lot.
> 
> So I have opened a series of smaller PRs: #138117 #138118 #138119 #138120
> 
> This branch is equal to the other ones (once they are merged together), so we can go either way. I think you can publish them from drafts once you are happy with them. You can also mention me on GH, so I get the notification instantly and publish them.

All these should be merged now, so this PR can probably be closed now.

https://github.com/llvm/llvm-project/pull/134494