http://askubuntu.com/questions/823646/how-can-i-install-clang-3-9-on-ubuntu-12-04

It seems you can install clang 3.9 on Ubuntu 12.04.

1 - You need to add these repository to your sources.list file.

deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty main
deb-src http://apt.llvm.org/trusty/ llvm-toolchain-trusty main
# 3.8 
 

deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-3.8 main
deb-src http://apt.llvm.org/trusty/ llvm-toolchain-trusty-3.8 main
# 3.9 
deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-3.9 main
deb-src http://apt.llvm.org/trusty/ llvm-toolchain-trusty-3.9 main
2- Then you need to add ubuntu-toolchain-r/test ppa. Because it
 
 provides updated libstdc++ required by clang 3.9

sudo add-apt-repository ppa:ubuntu-toolchain-r/test
3 - Now update the repository index

sudo apt-get update
4 - You should now be able to install clang 3.9

sudo apt-get install clang-3.9
Source: http://apt.llvm.org/