[clang][cli] Fix gcc warning (NFC)

GCC warning:
```
/llvm-project/clang/lib/Frontend/TestModuleFileExtension.cpp:131:20: warning: ‘llvm::raw_ostream& clang::operator<<(llvm::raw_ostream&, const clang::TestModuleFileExtension&)’ has not been declared within ‘clang’
  131 | llvm::raw_ostream &clang::operator<<(llvm::raw_ostream &OS,
      |                    ^~~~~
In file included from /llvm-project/clang/lib/Frontend/TestModuleFileExtension.cpp:8:
/llvm-project/clang/lib/Frontend/TestModuleFileExtension.h:75:3: note: only here as a ‘friend’
   75 |   operator<<(llvm::raw_ostream &OS, const TestModuleFileExtension &Extension);
      |   ^~~~~~~~
```