[clang] [analyzer] Fix assertion failure in `CXXInstanceCall::getCXXThisVal` (PR #70837)

[Balazs Benics via cfe-commits]

steakhal wrote:

> > WDYT?
> 
> I like this! I hope we do not add too much redundant work, but at least we make it clear what is the plan to fix this in the future.

Please approve the PR again, so that I could merge this after I give some time for others to look at this.

https://github.com/llvm/llvm-project/pull/70837