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

[Michael Spencer via cfe-commits]

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@@ -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
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Bigcheese wrote:

Should this just be "Reducing test cases"?

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