Personally I encountered this issue and solved it by installing LLVM using the Automatic installation script they mention in their apt packages page.

18.04 (Bionic)

I visited http://apt.llvm.org/bionic/dists/ (i.e. bionic distributions).
I determined that 6.0 was the latest major version of the toolchain.

I assume that you'll want the linker, lld, also.

# grab the key that LLVM use to GPG-sign binary distributions
wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
sudo apt-get update
sudo apt-add-repository "deb http://apt.llvm.org/bionic/ llvm-toolchain-bionic-6.0 main"
sudo apt-get install -y clang-6.0 lld-6.0

This gives you binaries with the following names (and more, probably):

clang-6.0
clang++-6.0
lld-6.0
ld.lld-6.0

It also installs these packages (and more):

llvm-6.0
llvm-6.0-dev
llvm-6.0-runtime

17.04 (Artful)

Same as above. I'll repeat every line for convenient copy-paste.

# grab the key that LLVM use to GPG-sign binary distributions
wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
sudo apt-get update
sudo apt-add-repository "deb http://apt.llvm.org/artful/ llvm-toolchain-artful-6.0 main"
sudo apt-get install -y clang-6.0 lld-6.0

16.04 (Xenial)

The accepted answer already gives instructions for installing clang-3.8 on 16.04, but here's how to get clang-6.0:

# grab the key that LLVM use to GPG-sign binary distributions
wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
sudo apt-get update
sudo apt-add-repository "deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-6.0 main"
sudo apt-get install -y clang-6.0 lld-6.0

I would suggest installing the latest stable 15

1. Add the following lines to your /etc/apt/sources.list:

Copydeb http://apt.llvm.org/bullseye/ llvm-toolchain-bullseye main
deb-src http://apt.llvm.org/bullseye/ llvm-toolchain-bullseye main
# 15
deb http://apt.llvm.org/bullseye/ llvm-toolchain-bullseye-15 main
deb-src http://apt.llvm.org/bullseye/ llvm-toolchain-bullseye-15 main

2. Add signatures for these repos (otherwise apt-get update will complain in the next step)

Copywget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -

3. Run apt-get update to add these new repos to the apt-get:

Copy sudo apt update && sudo apt upgrade

4. To install all key packages

Copysudo apt-get install libllvm-15-ocaml-dev libllvm15 llvm-15 llvm-15-dev llvm-15-doc llvm-15-examples llvm-15-runtime

5. Install clang-15

Copy sudo apt-get install clang-15 lldb-15 lld-15     

6. Make sure now "clang-15" is used by the compiler and not the older "clang" and done just compile your project

Copyexport CMAKE_C_COMPILER=clang-15

export CMAKE_CXX_COMPILER=clang++-15

Doc https://apt.llvm.org/