Use DIExpression::foldConstantMath() at the result of an append() (#71719)

This patch uses `DIExpression::foldConstantMath()` at the end of a
`DIExpression::append()`. Which should help in reducing the size of
DIExpressions that grow because of salvaging debug info

This is part of a stack of patches and comes after:
https://github.com/llvm/llvm-project/pull/69768
https://github.com/llvm/llvm-project/pull/71717
https://github.com/llvm/llvm-project/pull/71718