Disallow implicit conversions from None to integer types

This fixes an error introduced in r228934 where None was converted to
an int instead of the int being converted to an Optional as intended.
We make that sort of mistake a compile error by changing NoneType into
a scoped enum.

Finally, provide a static NoneType called None to avoid forcing all
users to spell it NoneType::None.

llvm-svn: 229980