Make the static instance of None just const.

This way there shouldn't be any unused variable warnings.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230010 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/None.h

@@ -20,7 +20,7 @@ namespace llvm {
 /// \brief A simple null object to allow implicit construction of Optional<T>
 /// and similar types without having to spell out the specialization's name.
 enum class NoneType { None };
-static NoneType None;
+const NoneType None = None;
 }
 
 #endif