[clang] [Clang][Cygwin] attempt to fix building shared libclang. (PR #138351)

[via cfe-commits]

jeremyd2019 wrote:

`-DLLVM_BUILD_LLVM_DYLIB=OFF -DLLVM_LINK_LLVM_DYLIB=OFF -DLIBCLANG_BUILD_STATIC=ON`

Now we know that adding `-DLLVM_ENABLE_PIC=OFF` will turn off building libclang.dll, but otherwise it will be built regardless.

https://github.com/llvm/llvm-project/pull/138351