[clang] [AST] Migrate away from PointerUnion::{is, get} (NFC) (PR #119523)

[Nikita Popov via cfe-commits]

nikic wrote:

It looks like this somehow negatively affects stage2 clang performance: https://llvm-compile-time-tracker.com/compare.php?from=00e1cc4c9d002c78cf890b630343b052ebca0399&to=624cc7048f604ed1087f63fdbe4cbf40f1d35b69&stat=instructions:u

https://github.com/llvm/llvm-project/pull/119523