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

[Jonas Hahnfeld via llvm-commits]

hahnjo wrote:

> Here is another example of a hack in a custom build of Clang that could be avoided with this patch

By the looks of it, this is the same issue that can be properly solved in our sources by using `DISABLE_LLVM_LINK_LLVM_DYLIB`.

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