[clang] [clang] C++20 Modules: document how to perform automated reductions (PR #124997)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

Yes, we don't have a generic document on automated reduction without modules.

The idea of this document here is to inform developers who already know how to perform such reductions, so that they can target modules as well.

The key piece of information here that surprises most people is that cvise supports multiple source files, and handling directories recursively, so you can just write an interestingness test that targets building a whole project using its own build system.

https://github.com/llvm/llvm-project/pull/124997