[RemoveDIs] Preserve debug info format in llvm-reduce (#89220)

As the goal of LLVM reduce is to simplify the input file, it should not
modify the debug info format - doing so by default would make it
impossible to reduce an error that only occurs in the old format, for
example (as briefly discussed at
https://github.com/llvm/llvm-project/pull/86275). This patch uses the
new "preserve debug info format" flag in llvm-reduce to prevent the
input from being subtly transformed by llvm-reduce itself; this has no
effect on any tools used for the interestingness check (i.e. if `opt` is
invoked, it will still convert the reduced input to the new format by
default), but simply ensures that the reduced file is strictly reduced
rather than modified.