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

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AdvenamTacet wrote:

>  I think we need to understand the problem better.

Agree as well, I changed PR to WIP. But I don't really know where to look now.

> I think popping into a debugger would likely help here.

@shafik  What is the correct way of building a single test, so we have access to a stand-alone binary?

I did want to use a debugger, copy pasted many dependencies into one file, it even compiled, but it didn't reproduce the error.

I'm happy to get some help here.

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