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

[Artem Dergachev via cfe-commits]

haoNoQ wrote:

Ok I've merged the fix from #179788 and I've included the change I left out of #179788 where the storage location is propagated for regular assignment too. This should be correct given that we've already been doing the same for compound assignment, even without ever creating a value.

So I think this patch is ready for review.

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