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

[Matheus Izvekov via llvm-commits]

https://github.com/mizvekov commented:

Can you test this on https://llvm-compile-time-tracker.com to spot any regressions?

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