Add a test to check that warning is gone

Author: sharadhr

clang/test/Driver/cl-cxx20-modules.cppm

@@ -1,3 +1,6 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
 // RUN: %clang_cl /std:c++20 --precompile -### -- %s 2>&1 | FileCheck --check-prefix=PRECOMPILE %s
 // PRECOMPILE: -emit-module-interface
 
@@ -6,3 +9,8 @@
 
 // RUN: %clang_cl /std:c++20 --fprebuilt-module-path=. -### -- %s 2>&1 | FileCheck --check-prefix=FPREBUILT %s
 // FPREBUILT: -fprebuilt-module-path=.
+
+// RUN: %clang_cl %t/test.pcm /std:c++20 -### 2>&1 | FileCheck --check-prefix=CPP20WARNING %t/test.pcm
+
+//--- test.pcm
+// CPP20WARNING-NOT: clang-cl: warning: argument unused during compilation: '/std:c++20' [-Wunused-command-line-argument]