[clang] [C++20][Modules][Serialization] Add an additional test case for #120277. (PR #126349)

[Chuanqi Xu via cfe-commits]

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@@ -0,0 +1,67 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+// RUN: cd %t
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ChuanqiXu9 wrote:

```suggestion

```

https://github.com/llvm/llvm-project/pull/126349