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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

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

Okay, sounds fair.

OptionalUnsigned implies less genericity than what's offered here, as we support enums with any unsigned underlying type as well.

But then `OptionalUnsignedUnderlying` sounds like quite a mouthful.

Any other opinions?

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