Allow Optionals to be compared to None

Allow Optionals to be compared to None

This is something like nullopt in std::experimental::optional. Optional
could already be constructed from None, so this seems like an obvious
extension from there.

I have a use in a future patch for Clang, though it may not go that
way/end up used - so this seemed worth committing now regardless.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@245518 91177308-0d34-0410-b5e6-96231b3b80d8