[ADT][NFC] Add missing closing comment for `llvm` namespace

[ken-matsui]

Summary

As clang-tidy suggested adding a closing comment for the llvm namespace, I added it.

[whisperity]

This is unrelated to Clang-Tidy, the change affects the LLVM ADT library.

(Also, what is the policy for commits like these? This seems to be like borderline noise, a simple formatting commit, nothing else.)

It is not achieving anything, but also breaks the formatting rules that are found in other parts of the project. For example, APInt.h is terminated with } // namespace llvm and not } // end namespace llvm, see http://github.com/llvm/llvm-project/blob/40a2e35599b5e5fa0dd93ac4edf4bf02ee636f6e/llvm/include/llvm/ADT/APInt.h#L2301.

[ken-matsui]

Oh, I didn't know their policy aren't unified and thought this file just wasn't adapted because we have .clang-tidy. Closing.

llvm/include/llvm/ADT/None.h

 #define LLVM_ADT_NONE_H
 
 namespace llvm {
 /// A simple null object to allow implicit construction of Optional<T>
 /// and similar types without having to spell out the specialization's name.
 // (constant value 1 in an attempt to workaround MSVC build issue... )
 enum class NoneType { None = 1 };
 const NoneType None = NoneType::None;
-}
+} // end namespace llvm
 
 #endif