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

[Gábor Horváth via cfe-commits]

Xazax-hun 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.

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