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

[Tomohiro Kashiwada via cfe-commits]

kikairoya wrote:

> > a symlink alone won't suffice here, as `dlopen()` ultimately calls `LoadLibraryExW`, which does not follow Cygwin-style symlinks.
> 
> If dlopen doesn't work on a Cygwin symlink, that is a bug in dlopen and should be fixed in Cygwin.

I had been misunderstood the functionality. That's cool. Thank you.

I made changes for description, commit and function body to use symlink.

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