[NFC] [C++20] [Modules] Adopt a test case from MSVC

Add a test case from MSVC
(https://developercommunity.visualstudio.com/t/c20-modules-unresolved-external-symbol/10049210)
to cover more tests.

clang/test/Modules/seperated-member-function-definition-for-template-class.cppm

@@ -0,0 +1,52 @@
+// This comes from the issue report of MSVC
+// (https://developercommunity.visualstudio.com/t/c20-modules-unresolved-external-symbol/10049210).
+//
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/base.cppm -emit-module-interface -o %t/package-base.pcm
+// RUN: %clang_cc1 -std=c++20 %t/child.cppm -emit-module-interface -o %t/package-child.pcm \
+// RUN:     -fprebuilt-module-path=%t
+// RUN: %clang_cc1 -std=c++20 %t/package.cppm -emit-module-interface -o %t/package.pcm \
+// RUN:     -fprebuilt-module-path=%t
+// RUN: %clang_cc1 -std=c++20 %t/use.cpp -fsyntax-only -verify -fprebuilt-module-path=%t
+
+//--- base.cppm
+export module package:base;
+
+export struct child;
+
+export
+template<class> struct base
+{
+	child getChild();
+};
+
+
+//--- child.cppm
+export module package:child;
+
+import :base;
+
+export struct child : base<void> {};
+
+template<class T>
+child base<T>::getChild() { return {}; }
+
+//--- package.cppm
+export module package;
+
+export import :base;
+export import :child;
+
+//--- use.cpp
+// expected-no-diagnostics
+import package;
+
+int use()
+{
+	base<void>{}.getChild();
+	base<int>{}.getChild();
+	return 0;
+}