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

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

> I'm fighting the tests on Windows - the hurd test I started from had `// UNSUPPORTED: system-windows` at the top, probably because they weren't interested in messing around with backslashes and .exe extensions. I don't know if the cross test will work right for the path to the cross as, is there some way to annotate just one test in the file as unsupported or expected fail or something on Windows?
> 
> (Locally, I ran the tests on Cygwin and they worked fine)
> 
> Looking at the output, it looks like maybe just a slash vs backslash again, so maybe it can be made to match on Windows

If it's just a slash issue, then applying copious amounts of regexes (e.g. `{{/\\\\}}` to match both) may help.

For skipping smaller sections of tests, you can apply `%if !system-windows %{ ... %}` around a bit of the RUN commands, see f5a93c5f2a4d0916c975bbf028768d58a29b6b73 for an example of that. If it's a bigger issue and relevant to most of a test file, it may be worth splitting into a separate file which can be marked `// UNSUPPORTED: system-windows`.

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