[llvm] Use DIExpression::foldConstantMath() at the result of an append() (PR #71719)

[Stephen Tozer via llvm-commits]

SLTozer wrote:

> I have updated this patch, I am not aware of how to push to the compile time tracker, can you please help me out

I can submit your branch to the compile time tracker, but it looks like the [final commit](https://github.com/llvm/llvm-project/pull/71721/commits/6e3b0eea6ff75637185a14c92602790d061b661e) and at least some of the preceding commits [fail when building](http://llvm-compile-time-tracker.com/show_error.php?commit=6e3b0eea6ff75637185a14c92602790d061b661e), so there's no measurement right now. If you get the patches building successfully with tests passing, I can resubmit it to get compile time stats.

https://github.com/llvm/llvm-project/pull/71719