Bug 1182996 - Fix and add missing namespace comments. rs=ehsan

The bulk of this commit was generated by running:

  run-clang-tidy.py \
    -checks='-*,llvm-namespace-comment' \
    -header-filter=^/.../mozilla-central/.* \
    -fix

xpcom/threads/ThreadStackHelper.cpp

@@ -217,7 +217,7 @@ public:
   ScopedSetPtr(T*& p, T* val) : mPtr(p) { mPtr = val; }
   ~ScopedSetPtr() { mPtr = nullptr; }
 };
-}
+} // namespace
 
 void
 ThreadStackHelper::GetStack(Stack& aStack)