[llvm-shlib] Change libLLVM-$MAJOR.so symlink to point to versioned SO (#82660)

This symlink was added in 91a38462 to
maintain backwards compatibility, but it needs to point to
libLLVM.so.$MAJOR.$MINOR rather than libLLVM.so. This works better for
distros that ship libLLVM.so and libLLVM.so.$MAJOR.$MINOR in separate
packages and also prevents mistakes like
libLLVM-19.so -> libLLVM.so -> libLLVM.so.18.1

Fixes #82647