Handle new LLVM soname

LLVM now includes the minor version in the soname, and also changed
the names of shared object files. libLLVM-18.so is now a symlink to
libLLVM.so.18.1. We need to make some changes to support this:

First, we need to run the installed llvm-config binary, rather
than the one from the build directory. This is because the symlink
does not exist in the build directory, but llvm-config requires it.
This looks like an LLVM bug to me, but it's probably a good idea to
use the installed version anyway.

Second, when installing LLVM into the libdir, we need to install
the target of the symlink, ans this is what will get loaded at
runtime.

However, the rust-dev component in particular also needs to
distribute the symlink itself, as download-ci-llvm will end up
invoking llvm-config, which requires the symlink to exist. The
symlink is not shipped in other components.