[C++20] [Modules] Add a test

A test from a regression in the downstream.

The test should be always good.

Author: ChuanqiXu9

clang/test/Modules/no-transitive-decl-change-3.cppm

@@ -0,0 +1,28 @@
+// Test that adding a new unused decl within reduced BMI may not produce a transitive change.
+//
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 -emit-reduced-module-interface %t/A.cppm -o %t/A.pcm
+// RUN: %clang_cc1 -std=c++20 -emit-reduced-module-interface %t/B.cppm -o %t/B.pcm \
+// RUN:     -fmodule-file=A=%t/A.pcm
+//
+// RUN: %clang_cc1 -std=c++20 -emit-reduced-module-interface %t/A.v1.cppm -o %t/A.v1.pcm
+// RUN: %clang_cc1 -std=c++20 -emit-reduced-module-interface %t/B.cppm -o %t/B.v1.pcm \
+// RUN:     -fmodule-file=A=%t/A.v1.pcm
+//
+// RUN: not diff %t/B.pcm %t/B.v1.pcm &> /dev/null
+
+//--- A.cppm
+export module A;
+export int a() { return 44; }
+
+//--- A.v1.cppm
+export module A;
+export int a() { return 44; }
+int a_impl() { return 48; }
+
+//--- B.cppm
+export module B;
+import A;
+export inline int b = a();