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

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

> > Then secondly, LLVM only allows "squash and merge" merging of PRs - so all these nicely split changes (with individual explanations soon, I'd hope!) would end up all mushed together. So unfortunately, to preserve the info about each of them, they'd need to be filed in separate individual PRs.
> 
> I think the descriptions would be preserved if I combined them all into PR description. I've split the commits only to make the review easier.

Ok, fair enough - we can keep this as one single PR. If the PR description contains all the info needed to cover the why/how for all the changes here that's probably fine too.

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