LLVM versions

See my [post](https://mail.mozilla.org/pipermail/rust-dev/2013-April/003518.html) on the mailing list for details.

Also:

* 3.2 is newer than 3.2svn, so I changed the ordering. (3.2svn is the version used between 3.2 and 3.3.)
* 3.3svn is added. This is the version you currently get from compiling LLVM trunk.
* 3.3 is added. 3.3 is not released yet, but this is the version used by http://llvm.org/apt/.