[clang] [clang][analyzer] Fix liveness and analysis of [[assume]] attributes (PR #198618)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

> Ask next time. I have studied that code quite a bit already.

I happened to be familiar with the concrete parts of `LiveVariables` that I needed to modify here (because I accidentally encountered it during some investigation a few weeks ago), but I don't have an overall familiarity with the area as a whole.

https://github.com/llvm/llvm-project/pull/198618