you can fin the path of both files by using the "whereis" command so for llvm@9 you type whereis llvm@9 and it will search for the file and display it's pathNot sure of source or destination path. I don't really know where llvm@9 and llvm11 downloaded or installed to.
Is this a path to a file or folder?then you do ln -s <path of llvm@9> <path of llvm11>