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

[Michael Spencer via cfe-commits]

================
@@ -2072,3 +2072,25 @@ Interoperability with Clang Modules
 We **wish** to support Clang modules and standard C++ modules at the same time,
 but the mixing them together is not well used/tested yet. Please file new
 GitHub issues as you find interoperability problems.
+
+Finding reduced test cases
+--------------------------
+
+When the user encounters a problem with the implementation of standard modules,
+it's helpful to attach a reduced test case to the issue report.
+
+This reduction is hard, as it can't avoid the dependency on multiple files,
+unlike most issues which can be reproduced within a single translation unit.
----------------
Bigcheese wrote:

Wording is a bit awkward here, maybe:

```suggestion
Reducing modules issues is complicated by the need to reduce multiple files at the same time, unlike non-modules issues which can generally be reproduced within a single translation unit.
```

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