Use DIExpression::foldConstantMath at the result of a Salvaged expression (#71721)

This patch uses `DIExpression::foldConstantMath()` at the result of a
Salvaged expression, that is, it runs the folding optimizations after an
expression has been salvaged completely, to reduce how many times the
fold optimization function is called. Which should help in reducing the
size of DIExpressions that grow because of salvaging debug info

After checking the size of the dSYM with and without this change, I saw
a decrease of about 300KB, where the debug_loc section is about 1.6 GB
in size.

Where the debug loc section reduced in size by 212KB and it is 193MB in
size, the rest comes from the debug_info section

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