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

4b35dd5
was shipped to address #120277 .

It was thought to be a regression in 19.x according to this comment:
#120277 (comment)

This is a test case that fails even in 17.x but nevertheless is also
fixed by: 4b35dd5

Author: mpark

clang/test/Modules/pr120277-2.cpp

@@ -0,0 +1,66 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -std=c++20 -emit-header-unit -xc++-user-header %t/hu-01.h \
+// RUN:  -o %t/hu-01.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-header-unit -xc++-user-header %t/hu-02.h \
+// RUN:  -Wno-experimental-header-units -fmodule-file=%t/hu-01.pcm -o %t/hu-02.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-header-unit -xc++-user-header %t/hu-03.h \
+// RUN:  -Wno-experimental-header-units \
+// RUN:  -fmodule-file=%t/hu-01.pcm -o %t/hu-03.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-header-unit -xc++-user-header %t/hu-04.h \
+// RUN:  -Wno-experimental-header-units -fmodule-file=%t/hu-02.pcm \
+// RUN:  -fmodule-file=%t/hu-03.pcm -o %t/hu-04.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-obj %t/main.cpp \
+// RUN:  -Wno-experimental-header-units -fmodule-file=%t/hu-04.pcm
+//--- hu-01.h
+template <typename T>
+struct A {
+  ~A() { f(); }
+  auto f() const { return 0; }
+};
+
+template <typename T>
+struct B {
+  int g() const { return a.f(); }
+  A<T> a;
+};
+
+//--- hu-02.h
+import "hu-01.h";
+
+template <typename = void>
+struct C {
+  void h() {
+    B<int>().g();
+  }
+};
+
+template struct A<double>;
+
+//--- hu-03.h
+import "hu-01.h";
+
+inline B<int> b() {
+  return {};
+}
+
+//--- hu-04.h
+import "hu-02.h";
+import "hu-03.h";
+
+inline void f4() {
+  C{}.h();
+}
+
+//--- main.cpp
+import "hu-04.h";
+
+int main() {
+  f4();
+}