To get LLVM clang working on macOS 10.14.1 (Mojave), install the developer command-line tools:
assuming you first installed Xcode from the App Store.
This has been the only thing required for ages to get LLVM working on macOS.
However for some reason things have changed on Mojave and we now have to install
essential headers like
wchar_t.h in /usr/include with an extra step. Just run
this command and go through the installation wizard: