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

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https://github.com/NeKon69 approved this pull request.

> 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 :)

+1
Either way, I'm fine with merging this as is

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