[Driver] [C++20] [Modules] Don't emit unused-argument warning for ` -fmodule-output=` and `-fmodule-output`

[ChuanqiXu]

Summary

Suppres the r unused-argument warning for -fmodule-output according to the discussion thread in https://gcc.gnu.org/pipermail/gcc/2022-December/240239.html.

[dblaikie]

Looks good, thanks! (for the record I'm not 100% sure this is the right direction (I wonder if this'll be confusing for users if they ask for the module output but didn't get any because they weren't doing a modular build, but that's not obvious, etc), so I don't mind if, in retrospect, we find that unused command line warning turns out to be suitable and revert this patch some time in the future - but I think "the same as -o" is a good place to start, at least)

clang/lib/Driver/ToolChains/Clang.cpp

if (HaveClangModules) {
     Args.ClaimAllArgs(options::OPT_fbuild_session_timestamp);
     Args.ClaimAllArgs(options::OPT_fbuild_session_file);
     Args.ClaimAllArgs(options::OPT_fmodules_validate_once_per_build_session);
     Args.ClaimAllArgs(options::OPT_fmodules_validate_system_headers);
     Args.ClaimAllArgs(options::OPT_fno_modules_validate_system_headers);
     Args.ClaimAllArgs(options::OPT_fmodules_disable_diagnostic_validation);
   }
 
+  // Claim `-fmodule-output` and `-fmodule-output=` to avoid unused warnings.
+  Args.ClaimAllArgs(options::OPT_fmodule_output);
+  Args.ClaimAllArgs(options::OPT_fmodule_output_EQ);
+
   return HaveModules;
 }
 
 static void RenderCharacterOptions(const ArgList &Args, const llvm::Triple &T,
                                    ArgStringList &CmdArgs) {
   // -fsigned-char is default.
   if (const Arg *A = Args.getLastArg(options::OPT_fsigned_char,
                                      options::OPT_fno_signed_char,

clang/test/Driver/module-output.cppm

 // multiple archs.
 // RUN: %clang %t/Hello.cppm -fmodule-output -arch i386 -arch x86_64 -### -target \
 // RUN:   x86_64-apple-darwin 2>&1 | FileCheck %t/Hello.cppm -check-prefix=MULTIPLE-ARCH
 
 // Tests that the .pcm file will be generated in the same path with the specified one
 // in the comamnd line.
 // RUN: %clang -std=c++20 %t/Hello.cppm -fmodule-output=%t/pcm/Hello.pcm -o %t/Hello.o \
 // RUN:   -c -### 2>&1 | FileCheck %t/Hello.cppm --check-prefix=CHECK-SPECIFIED
+//
+// RUN: %clang -std=c++20 %t/Hello.cppm -fmodule-output=%t/Hello.pcm -fmodule-output -c -fsyntax-only \
+// RUN:   -### 2>&1 | FileCheck %t/Hello.cppm --check-prefix=CHECK-NOT-USED
 
 //--- Hello.cppm
 export module Hello;
 
 // CHECK: "-emit-module-interface" {{.*}}"-main-file-name" "Hello.cppm" {{.*}}"-o" "{{.*}}/output/Hello.pcm" "-x" "c++" "{{.*}}/Hello.cppm"
 // CHECK: "-emit-obj" {{.*}}"-main-file-name" "Hello.cppm" {{.*}}"-o" "{{.*}}/output/Hello.o" "-x" "pcm" "{{.*}}/output/Hello.pcm"
 
 // MULTIPLE-ARCH: option '-fmodule-output' can't be used with multiple arch options
 
 // CHECK-SPECIFIED: "-emit-module-interface" {{.*}}"-main-file-name" "Hello.cppm" {{.*}}"-o" "{{.*}}/pcm/Hello.pcm" "-x" "c++" "{{.*}}/Hello.cppm"
 // CHECK-SPECIFIED: "-emit-obj" {{.*}}"-main-file-name" "Hello.cppm" {{.*}}"-o" "{{.*}}/Hello.o" "-x" "pcm" "{{.*}}/pcm/Hello.pcm"
 
+// CHECK-NOT-USED-NOT: warning: argument unused during compilation: '-fmodule-output'
+// CHECK-NOT-USED-NOT: warning: argument unused during compilation: '-fmodule-output=Hello.pcm'
+// CHECK-NOT-USED-NOT: "-fmodule-output"
+// CHECK-NOT-USED-NOT: "-fmodule-output="
+
 //--- AnotherModule.cppm
 export module AnotherModule;
 // CHECK: "-emit-module-interface" {{.*}}"-main-file-name" "Hello.cppm" {{.*}}"-o" "{{.*}}/Hello.pcm" "-x" "c++" "{{.*}}/Hello.cppm"
 // CHECK: "-emit-obj" {{.*}}"-main-file-name" "Hello.cppm" {{.*}}"-o" "{{.*}}/Hello-{{.*}}.o" "-x" "pcm" "{{.*}}/Hello.pcm"
 // CHECK: "-emit-module-interface" {{.*}}"-main-file-name" "AnotherModule.cppm" {{.*}}"-o" "{{.*}}/AnotherModule.pcm" "-x" "c++" "{{.*}}/AnotherModule.cppm"
 // CHECK: "-emit-obj" {{.*}}"-main-file-name" "AnotherModule.cppm" {{.*}}"-o" "{{.*}}/AnotherModule-{{.*}}.o" "-x" "pcm" "{{.*}}/AnotherModule.pcm"