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

[Mateusz Mikuła via cfe-commits]

mati865 wrote:

So this is basically `hack: dynamically resolve G++ include dir` and `hack: add system search paths` from https://github.com/mati865/llvm-project/commits/cygwin-more-fixes/ but done the proper way.

It will take a few more changes to get everything working, but this is a good start.

LGTM

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