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

[徐持恒 Xu Chiheng via cfe-commits]

xu-chiheng wrote:

> > 
> > 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.

The original Generic_GCC driver can be built on Cygwin, but does not work on non trivial cases.
Even Cygwin's old outdated llvm patches use a driver modified from MinGW driver .
https://cygwin.com/git-cygwin-packages/
https://cygwin.com/git-cygwin-packages/?p=git/cygwin-packages/clang.git;a=summary
git://cygwin.com/git/cygwin-packages/clang.git
https://cygwin.com/git-cygwin-packages/?p=git/cygwin-packages/llvm.git;a=summary
git://cygwin.com/git/cygwin-packages/llvm.git


Combining Cygwin and MinGW driver is simple. But the combined code may be ugly.
 



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