[lld] Migrate away from PointerUnion::dyn_cast (NFC)#123691

kazutakahirata wants to merge 2 commits intollvm:mainllvm/llvm-project:mainfrom

kazutakahirata:cleanup_001_PointerUnion_lldkazutakahirata/llvm-project:cleanup_001_PointerUnion_lldCopy head branch name to clipboard