[clang] [clang] NFC: Refactor UnsignedOrNone into SmallOptional<T> with enum support (PR #191828)

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin commented:

Can we call that `OptionalUnsigned` or something ? SmallOptional implies a genericity that this does not have. Thanks

https://github.com/llvm/llvm-project/pull/191828