How to install LLVM for Mac?

Remember to source .bash_profile! I wasted a lot of time due to the silly problem.

homebrew does not link llvm to /usr/local/bin because it may conflict with the system one, causing all kinds of nasty bugs. Instead, you should use the full path to invoke them, such as /usr/local/opt/llvm/bin/lli.

In fact there's a Caveat that's now listed which spells this out...

==> Caveats
==> llvm

To use the bundled libc++ please add the following LDFLAGS:
LDFLAGS="-L/usr/local/opt/llvm/lib -Wl,-rpath,/usr/local/opt/llvm/lib"

llvm is keg-only, which means it was not symlinked into /usr/local, because macOS already provides this software and installing another version in parallel can cause all kinds of trouble.

If you need to have llvm first in your PATH run:
echo 'export PATH="/usr/local/opt/llvm/bin:$PATH"' >> ~/.bash_profile

For compilers to find llvm you may need to set:
export LDFLAGS="-L/usr/local/opt/llvm/lib"
export CPPFLAGS="-I/usr/local/opt/llvm/include"

You can also add /usr/local/opt/llvm/bin/ to your $PATH environment variable:

echo 'export PATH="/usr/local/opt/llvm/bin:$PATH"' >> ~/.bash_profile
source ~/.bash_profile

This way you can call other llvm commands such as llvm-config. A list of llvm can be found here.