[clang] [clang] Replace PointerUnion::dyn_cast with llvm::dyn_cast (NFC) (PR #232603)

[Nikita Popov via cfe-commits]

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

LGTM

For what it's worth, I do think it would be fine to mass-migrate these. The fact that the tests don't crash is already pretty good proof... unless we're missing test coverage, in which case it's an opportunity to discover missing tests :)

https://github.com/llvm/llvm-project/pull/232603