[llvm] Use DIExpression::foldConstantMath at the result of a Salvaged expression (PR #71721)

[J. Ryan Stinnett via llvm-commits]

jryans wrote:

I agree with @adrian-prantl, a rewrite rule design should work quite well here. It should make this system easier to maintain and understand for long term.

Of course, that’s future work for after this PR, but I am happy to hear people are thinking along these lines. 😄

https://github.com/llvm/llvm-project/pull/71721