[clang] Extend life of variables in `DiagComparison` in `ExprConstant` (PR #79522)

[via cfe-commits]

AdvenamTacet wrote:

I rewrote all functions with turned off instrumentation (I removed `_LIBCPP_STRING_INTERNAL_MEMORY_ACCESS` completely) and I see no change in behavior. Error happens in the very same way with the very same shadow memory (and backtrace).
I don't see where annotations can be broken, I'm trying to find a place where annotations are updated incorrectly, but I don't see the place.

I'm also trying to reproduce the error with stand alone code, but without luck so far.

So it makes me think that the bug may be completely different and unrelated, but I don't know where to look next.
I don't like the idea of temporary solution, but I have no better suggestion at the moment. For a moment, I assume that annotations work correctly. 

https://github.com/llvm/llvm-project/pull/79522