[llvm] Reland "[llvm][ADT] Refactor PointerUnion to use PunnedPointer. NFC." (PR #188242)

[Jakub Kuderski via llvm-commits]

kuhar wrote:

I think it's technically wrong to lie about how many bits you have available, but I can see it being useful if you control the allocations and know you can align more than the type definition itself implies. So I'm inclined to keep it as an NFC for compatibility reasons because the workaround is straightforward.

https://github.com/llvm/llvm-project/pull/188242