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

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jeremyd2019 wrote:

> I see that that patch also contains some similar cleanups to InitHeaderSearch.cpp - do these patches share some common ancestry (e.g. some cygwin specific clang patchsets that have been floating around somewhere?).

No, I wasn't aware of that PR, I noticed that using Generic_GCC directly wasn't ever initing GCCInstallation member, and figured I had to add a derived class to do that to get rid of some of the "hacks" patches injecting paths elsewhere.

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