[clang] [clang][dataflow] Add basic modeling for compound assignments. (PR #179058)

[Artem Dergachev via cfe-commits]

haoNoQ wrote:

Gentle ping.

(I'm confident in my code and it's simple enough but I feel like I haven't quite demonstrated that I know what I'm doing. So it's probably a good idea to have somebody double-check the overall idea as it diverged after the rebase.)

https://github.com/llvm/llvm-project/pull/179058