[Serialization] Migrate away from PointerUnion::dyn_cast (NFC)#124676

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

kazutakahirata:cleanup_001_PointerUnion_clang_Serialization_ASTReaderDeclkazutakahirata/llvm-project:cleanup_001_PointerUnion_clang_Serialization_ASTReaderDeclCopy head branch name to clipboard