As reported by @jbsolomon-fw in #4799, when building on macOS with LLVM 10.0.1 installed via Homebrew, linker trips with the following error:

ld: library not found for -llibxml2.tbd

This seems like an upstream problem with llvm-config which reports the following required system libraries to link properly with LLVM:

$ llvm-config --system-libs
-lz -lcurses -lm -llibxml2.tbd