[Sema] Migrate away from PointerUnion::dyn_cast (NFC)#123158

kazutakahirata merged 1 commit intollvm:mainllvm/llvm-project:mainfrom

kazutakahirata:cleanup_001_PointerUnion_Semakazutakahirata/llvm-project:cleanup_001_PointerUnion_SemaCopy head branch name to clipboard