[ADT] Remove None=1 MSVC workaround

This workaround from 0cd22f9 (2017-01) is very
likely unneeded.

llvm/include/llvm/ADT/None.h

@@ -19,8 +19,7 @@
 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 };
+enum class NoneType { None };
 const NoneType None = NoneType::None;
 }