LLVM APT authentication problem

I'm using Ubuntu 16.04 and I can't install the 3.9 packages without
ignoring the authentication issue:

$ sudo apt-get install clang-3.9
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
  libclang-common-3.9-dev libclang1-3.9 libffi-dev libllvm3.9 libtinfo-dev
  llvm-3.9 llvm-3.9-dev llvm-3.9-runtime
Suggested packages:
  gnustep gnustep-devel clang-3.9-doc llvm-3.9-doc
The following NEW packages will be installed:
  clang-3.9 libclang-common-3.9-dev libclang1-3.9 libffi-dev libllvm3.9
  libtinfo-dev llvm-3.9 llvm-3.9-dev llvm-3.9-runtime
0 upgraded, 9 newly installed, 0 to remove and 2 not upgraded.
Need to get 66,6 MB/66,7 MB of archives.
After this operation, 334 MB of additional disk space will be used.
Do you want to continue? [Y/n] y
WARNING: The following packages cannot be authenticated!
  libllvm3.9 libclang1-3.9 libclang-common-3.9-dev clang-3.9 llvm-3.9-runtime
  llvm-3.9 llvm-3.9-dev
Install these packages without verification? [y/N] n
E: Some packages could not be authenticated

I had the same issue with Ubuntu 15.10 and reported here:

Is that intentional?