In the scope document for WRITING-8441, a question was raised concerning the use of Clang modules to improve build times in a manner similar to PCH. The suggestion was out of scope for the experiment we would like to run, but it seems like something we could consider enabling for local builds either if PCH doesn't work out or if there's an additional improvement beyond PCH for local builds. We will have to run IWYU across the codebase first, though, since using Clang modules requires the components to be independently buildable.