[clang] [Clang] [Driver] add a Cygwin ToolChain (PR #135691)

[via cfe-commits]

jeremyd2019 wrote:

To me, the general theory is that Cygwin is more UNIXy than Windowsy, so I based this off of actually the Hurd.h/cpp (as it had pretty limited amounts of special cases).

It looks to me like the #74933 PR followed the from-scratch ToolChain idea, and that probably accounts for why it was much more code.  I can look at that more closely and see if there's anything I'm missing.

I believe cygwin target testing is very lacking/non-existent.  I don't have experience with llvm's test suite though, but if it's just a matter of adding additional cases to existing tests or maybe copying some existing tests I might be OK figuring it out.

https://github.com/llvm/llvm-project/pull/135691