[TableGen] Migrate away from PointerUnion::dyn_cast (NFC)#125158

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

kazutakahirata:cleanup_001_PointerUnion_clang_TableGenkazutakahirata/llvm-project:cleanup_001_PointerUnion_clang_TableGenCopy head branch name to clipboard