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

[Balazs Benics via cfe-commits]

steakhal wrote:

> Hmm, I wonder if we should leave a FIXME comment, but it looks good to me.

I was thinking where to put the FIXME, and as I explored that should be within the CastVisitor. After that, I argued, that then I should still have the (ineffective) `SVB.evalCast()` to actually exercise that path.
It should be a fancy way of returning `Unknown` now, but arguably for the better reason xD

@Xazax-hun WDYT?

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