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

1 files changed, 4 insertions, 2 deletions

diff --git a/clang/lib/Frontend/TestModuleFileExtension.cpp b/clang/lib/Frontend/TestModuleFileExtension.cpp
index f7d3f15..0460969 100644
--- a/clang/lib/Frontend/TestModuleFileExtension.cpp
+++ b/clang/lib/Frontend/TestModuleFileExtension.cpp
@@ -128,9 +128,11 @@ TestModuleFileExtension::createExtensionReader(
                                                     new TestModuleFileExtension::Reader(this, Stream));
 }
 
-llvm::raw_ostream &clang::operator<<(llvm::raw_ostream &OS,
-                                     const TestModuleFileExtension &Extension) {
+namespace clang {
+llvm::raw_ostream &operator<<(llvm::raw_ostream &OS,
+                              const TestModuleFileExtension &Extension) {
   return OS << Extension.BlockName << ":" << Extension.MajorVersion << ":"
             << Extension.MinorVersion << ":" << Extension.Hashed << ":"
             << Extension.UserInfo;
 }
+} // namespace clang