[clang][CodeGen] Simplify code based on opaque pointers (#65624)

- Update CodeGenTypeCache to use a single union for all pointers in
  address space zero.
- Introduce a UnqualPtrTy in CodeGenTypeCache, and use that (for
  example instead of llvm::PointerType::getUnqual) in some places.
- Drop some redundant bit/pointers casts from ptr to ptr.