[llvm] [LLVM][Cygwin] Fix shared library name (PR #136599)

[via llvm-commits]

jeremyd2019 wrote:

After this PR, what you get is

* bin/  
  * cygclang-20.1.dll
  * cygclang-cpp-20.1.dll
  * cygLLVM-20.dll
  * cygRemarks-20.1.dll
* lib/  
  * libclang.dll.a
  * libclang-cpp.dll.a
  * libLLVM-20.dll.a
  * libRemarks.dll.a

and llvm-config --libs --link-shared returns `-lLLVM-20` as it should.


https://github.com/llvm/llvm-project/pull/136599