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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 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.

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