[clang] [libclang][Cygwin] Provide unversioned DLL file alongside versioned one (PR #147132)

[Tomohiro Kashiwada via cfe-commits]

kikairoya wrote:

I learned that a symlink works fine, so adding it to install-target might be a good idea. I'll look into that later.

https://github.com/llvm/llvm-project/pull/147132