$ sudo apt-get update
$ sudo apt-get install llvm-3.5 

llvm-3.5 is installed in your system.

Make ensure the llvm-3.5 package were installed using the commands given below,

$ sudo dpkg-query -l | grep llvm-3.5 *

You will get with llvm-3.5 package name, version, architecture and description in a table

llvm-3.5 package basic information: