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

[Fangrui Song via cfe-commits]

https://github.com/MaskRay commented:

LGTM. But I hope folks more familiar with Windows can take a look.
Can you describe the support triples in the description and state why we need the msys ones?

(Wow, MSYS and Cygwin... familiar names! I used them back in 2009, but then I made the switch to Linux.)

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