Tag the end of an anonymous namespace, NFC

Prevent clang-format from moving the closing branch.

llvm-svn: 267269
diff --git a/llvm/unittests/IR/MetadataTest.cpp b/llvm/unittests/IR/MetadataTest.cpp
index 1addd15..41b2be6 100644
--- a/llvm/unittests/IR/MetadataTest.cpp
+++ b/llvm/unittests/IR/MetadataTest.cpp
@@ -2309,4 +2309,4 @@
   EXPECT_EQ(SP, F->getMetadata(LLVMContext::MD_dbg));
 }
 
-}
+} // end namespace