$ sudo apt-get update
$ sudo apt-get install llvm-gcc-4.8 

llvm-gcc-4.8 is installed in your system.

Make ensure the llvm-gcc-4.8 package were installed using the commands given below,

$ sudo dpkg-query -l | grep llvm-gcc-4.8 *

You will get with llvm-gcc-4.8 package name, version, architecture and description in a table

llvm-gcc-4.8 package basic information: