[llvm] [ADT] Make null PointerUnion with different active members compare equal (PR #121847)

[Sergei Barannikov via llvm-commits]

s-barannikov wrote:

> Do we have an example of similar semantics in some other LLVM type or an external library?

`std::optional` with polymorphic type comes to mind.


https://github.com/llvm/llvm-project/pull/121847