[clang] [LifetimeSafety] Fix crash on ternary operator when one of the expressions is `throw` (PR #190345)

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NeKon69 wrote:

> Is this the part which is not working for you ?

What I was struggling with was something like what `isInevitablySinking` does (I had no idea that API existed at the time). The CFGBlock for the true / false branch / ternary operator itself would sometimes not point to the exact expressions, and because of that I had to go through each successor block from that block, etc. Since I am not really experienced with the codebase, I couldn't figure out how to do that correctly.

https://github.com/llvm/llvm-project/pull/190345