autoconf: Fix soname for libLLVM-Major.Minor.so

We were using libLLVM-Major.Minor.Patch.so for the soname, but we
need the soname to stay consistent for all Major.Minor.* releases
otherwise operating system distributors  will need to rebuild all
packages that link with LLVM every time there is a new point release.

This patch also reverses the compatibility symlink, so
libLLVM-Major.Minor.Patch.so is now a symlink that points
to libLLVM-Major-Minor.so.