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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

This probably needs a better justification in the commit message, plus probably should live in STLExtras instead of here.

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