[clang] [StaticAnalyzer] Migrate away from PointerUnion::{is,get} (NFC) (PR #118421)

[Balazs Benics via cfe-commits]

https://github.com/steakhal requested changes to this pull request.

Hi Kazu
I always welcome your patches! True gems.

I left a couple of comments, mostly about following llvm style guides.

https://github.com/llvm/llvm-project/pull/118421