What is the canonical directory in which to install parallel versions of LLVM?
There is no real standard.

Your choice of your home directory is easiest for compiling them. The only comment here is why the svn part of the directory I would just use ~/llvm. 

If you are the only user that the home directory makes sense however if your system is multi-user then they could be placed in /usr/local/llvm. As Unix is multi user and I suspect before OSX very few machines had one user the Unix standard is /usr/local and even now systems administrators and distributions have to assume multi user uise.

I would also consider using package managers like Macports or Homebrew. From reading Macports mailing lists compiling and using llvm is not just a simple compile and the porters wioll have worked out the issues or can work with you to fix problems.