[clang] [analyzer] Do not use APInt methods on _BitInt() Types (PR #65887)

[Balazs Benics via cfe-commits]

steakhal wrote:

I guess this is one sideeffect of `getZExtValue` and `getSExtValue`.
To me, it feels like all such calls could hit the same assert, thus it fixes this instance, but we still lack a generic solution to this problem at other places.
I'm not opposing to this fix, but it might make sense to step back and grep for these calls to see the wider picture.

https://github.com/llvm/llvm-project/pull/65887