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

[Jakub Kuderski via llvm-commits]

kuhar wrote:

BTW, I also remember that Rust has a similar concept called 'niche values' in case this help inspire naming/documentation: https://www.0xatticus.com/posts/understanding_rust_niche/  

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