`windows-clang` bring-your-own libclang

We need to make sure that it is easy to point it to your own build of libclang or at least clearly document how this would be done.

The issue names the windows-clang setup but no file or test. Start by tracing the windows-clang entry point and existing configuration documentation to see how libclang is selected. Make the user-built libclang workflow clear or straightforward, then verify the instructions or setup on Windows.

Written by the indexing model from the issue text.