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

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar approved this pull request.

This makes sense to ne. IMO having two uses (one in aarch, one in clang) is just about meets the threshold of this being general enough to go to ADT. I'm not sure about some implementation details like the assignment of sentinel being disallowed or lack of dense map info, but this seems like something we can hash out later 

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