Build with pre-installed LLVM on Windows

The readme contains some information on building with a pre-installed version of llvm/clang installed with apt-get on linux. I'm wanting to build the translator on Windows with a pre-installed LLVM/Clang, but can't find any information on it. Is there anyway to get cmake to point to the binaries in my llvm install, or is this version just missing critical cmake files?

Start with the README's existing instructions for building with a pre-installed LLVM/Clang and inspect the CMake configuration used by the translator. Determine whether an installed Windows LLVM/Clang can be selected through the current configuration or whether required CMake files are missing. Done means documenting the supported setup and the necessary CMake invocation, or clearly recording the limitation.

Written by the indexing model from the issue text.