Trying to access [LLVM Debian/Ubuntu nightly packages](http://apt.llvm.org/) 

I am on 14.04 and they give this info:

> 
> The goal is to provide Debian and Ubuntu nightly packages ready to be
>  installed with minimal impact on the distribution. Packages are
>  available for amd64 and i386 and for both the stable, qualification
>  and development branches (currently 3.9, 4.0 and 5.0).
> 
> 
> The packages provide LLVM + Clang + compiler-rt + polly + LLDB + LLD +
>  libFuzzer
> 
> 
> 

```
Trusty (14.04) - Last update : Wed, 22 Mar 2017 08:38:03 UTC / Revision: 298477

deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty main
deb-src http://apt.llvm.org/trusty/ llvm-toolchain-trusty 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
# 4.0 
deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-4.0 main
deb-src http://apt.llvm.org/trusty/ llvm-toolchain-trusty-4.0 main

```

I have tried going in Ubuntu Software Centre and adding the deb line for both 3.9 and 4.0. 
But after `sudo apt-get update` I get an error:

```
W: GPG error: http://apt.llvm.org llvm-toolchain-trusty-3.9 InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 15CF4D18AF4F7421

```

There is some way to install this without Ubuntu Software Centre, but I don't know how to find the ppa:user/ppa-name as talked about [here](https://help.ubuntu.com/community/Repositories/Ubuntu)