[clang] [llvm] [ADT] Add and use (for AArch64) `ValueOrSentinel<T, Sentinel>` (PR #158120)

[Matheus Izvekov via llvm-commits]

mizvekov wrote:

> I've tweaked `ValueOrSentinel` to allow passing an "adjustment" which allows it to handle use cases like `UnsignedOrNone`.

It seems like UnsignedOrNone is the only use case where we care about what the internal representation looks like.

And it seems like the other users also don't care about negative values, and it looks like they don't care about negative values either.

So can't we just use the UnsignedOrNone implementation throughout here?

https://github.com/llvm/llvm-project/pull/158120