[clang] Migrate away from PointerUnion::dyn_cast (NFC)#124425

kazutakahirata merged 2 commits intollvm:mainllvm/llvm-project:mainfrom

kazutakahirata:cleanup_001_PointerUnion_clang_nullkazutakahirata/llvm-project:cleanup_001_PointerUnion_clang_nullCopy head branch name to clipboard