[NFC] [C++20] [Modules] Tests that the duplicated declarations in GMF

won't get generated again

As the test shows, we don't want to see the specialized function bodies
if it is already contained in the imported modules. So we can save a lot
of compiling time then.

Author: ChuanqiXu9

clang/test/Modules/no-duplicate-codegen-in-GMF.cppm

@@ -0,0 +1,40 @@
+// Tests that the declaration won't get emitted after being merged.
+//
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple %t/A.cppm -emit-module-interface -o %t/A.pcm
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple %t/B.cppm -emit-module-interface -o %t/B.pcm \
+// RUN:     -fprebuilt-module-path=%t
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple %t/B.pcm -S -emit-llvm -o - | FileCheck %t/B.cppm
+
+//--- foo.h
+
+template <class T>
+class foo {
+public:
+    T value;
+    T GetValue() { return value; }
+};
+
+template class foo<int>;
+
+//--- A.cppm
+module;
+#include "foo.h"
+export module A;
+export using ::foo;
+
+//--- B.cppm
+module;
+#include "foo.h"
+export module B;
+import A;
+export using ::foo;
+export int B() {
+    foo<int> f;
+    return f.GetValue();
+}
+
+// CHECK-NOT: define{{.*}}@_ZN3fooIiE8GetValueEv