[clang] [Cygwin] Cygwin driver (PR #74933)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

> > @carlo-bramini has spent some effort on using Clang in Cygwin environments before, so as far as I know, it does work in general from before. So this change, which adds an entirely new driver for Cygwin environments, would need to be explained why it does that (I don't disagree, it's probably the right thing to do in general), how things worked before and how this changes things. And I would like to have @carlo-bramini's eye on this (and all the related Cygwin patches from @xu-chiheng).
> > And changes like this need some general tests, have a look at `clang/test/Driver` for how other drivers are tested.
> 
> The Cygwin driver is basically the same as MinGW driver with minor difference.

Right. What are the actual observable differences to what was executed before? (I presume this before used the `Generic_GCC` toolchain?) Also, I wonder if it would make sense to share the MinGW driver code with Cygwin, and just add exceptions where necessary, instead of duplicating it into a separate one? Maybe, but perhaps not.

https://github.com/llvm/llvm-project/pull/74933