[clang] [clang] NFC: introduce UnsignedOrNone as a replacement for std::optional<unsigned> (PR #134142)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

> Just noticed that this is not using `std::optional<unsigned>`. If this is good, why don't we put this to `llvm/ADT`? I feel that is a better place.

I don't disagree in principle, we just don't need to preempt making this available for all of llvm, if no one has the time to make a patch to use it outside of clang.

https://github.com/llvm/llvm-project/pull/134142