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

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https://github.com/cor3ntin requested changes to this pull request.

We need to understand what's going on here.
I suspect something gets tripped by the ternary.
A temporary fix might be to  assign 

```cpp
std::string first  = Reversed ? RHS : LHS;
std::string second  = Reversed ? LHS : RHS;
```
And use that in the diag.

We should also add a `std::string&&` overload to the diagnostic engine's `operator <<` to avoid extra copies, although that's mostly a luxury

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