[clang] [analyzer] Discard non-live source frames from the current stack (PR #213779)

[Benedek Kaibas via cfe-commits]

benedekaibas wrote:

> BTW is there a reason for spelling the title as `non-live source frames` and not as `dead source frames`?

I wrote `non-live` because the checker has to discard stack frames that are not on the current live stack. `dead` would imply that those stack frames were already analyzed and discarded, at least for me. Also `dead` refers to symbols that are no longer referenced and get removed by `checkDeadSymbols()` which can cause misunderstanding. Maybe `Discard stack frames that are not on the current live stack` would be the most accurate framing. 

https://github.com/llvm/llvm-project/pull/213779