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);
| ^~~~~~~~
```
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