Fixed/added namespace ending comments using clang-tidy. NFC

The patch is generated using this command:

tools/clang/tools/extra/clang-tidy/tool/run-clang-tidy.py -fix \
  -checks=-*,llvm-namespace-comment -header-filter='llvm/.*|clang/.*' \
  llvm/lib/


Thanks to Eugene Kosov for the original patch!

llvm-svn: 240137
diff --git a/llvm/lib/Support/RWMutex.cpp b/llvm/lib/Support/RWMutex.cpp
index 3b6309c..21ba5a4 100644
--- a/llvm/lib/Support/RWMutex.cpp
+++ b/llvm/lib/Support/RWMutex.cpp
@@ -113,7 +113,7 @@
   return errorcode == 0;
 }
 
-}
+} // namespace llvm
 
 #elif defined(LLVM_ON_UNIX)
 #include "Unix/RWMutex.inc"