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

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

Oh, also, what's the tradeoff between having this extend the `Generic_GCC` driver, vs doing a plain from-scratch implementation that extents the `ToolChain` base class like the `MinGW` driver does? I guess that it's a smaller step to just tweak `Generic_GCC`, not requiring all the same amount of boilerplate, potentially?

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