[clang] [NFC][analyzer] Refactor switch handling in the engine (PR #178678)

[Balázs Benics via cfe-commits]

steakhal wrote:

> I'm pretty confident that this patch is NFC (I paid attention to maintaining behavior when I changed things), but I will also test it by evaluating it on our usual test suite of ~a dozen open source projects to see that it doesn't change anything.
> 
> (Also, I'll remove that unused private field.)

At your discretion.

https://github.com/llvm/llvm-project/pull/178678