[llvm] [CMake] Only export the LLVM_LINK_LLVM_DYLIB setting if not yet set (PR #135570)

[Jonas Rembser via llvm-commits]

guitargeek wrote:

Ping.

Here is another example of a hack in a custom build of Clang that could be avoided with this patch:
https://github.com/conda-forge/root-feedstock/blob/main/recipe/build_root.sh#L185
```cmake
    # Cling needs some minor patches to the LLVM sources, hackily apply them rather than rebuilding LLVM
    sed -i "s at LLVM_LINK_LLVM_DYLIB yes at LLVM_LINK_LLVM_DYLIB no at g" "${Clang_DIR}/lib/cmake/llvm/LLVMConfig.cmake"
```

https://github.com/llvm/llvm-project/pull/135570