[clang] [clang] Replace PointerUnion::dyn_cast with llvm::dyn_cast (NFC) (PR #235232)

[Nikita Popov via cfe-commits]

nikic wrote:

Small compile-time improvement as a side-effect: https://llvm-compile-time-tracker.com/compare.php?from=f45474e16eb99c07e6b34cee08d26b44c8ce4148&to=ee824d2baaf7f864520141c0b5d3f704ed518985&stat=instructions:u

https://github.com/llvm/llvm-project/pull/235232