[llvm-exegesis] Add debug option to print per-measurement values (#81219)

This patch adds a debug option to print per measurement latency and
validation counter values. This makes it easier to debug certain
transient issues that can be hard to spot just using the summary view at
the end.

I've hacked print statements into this part of the code base enough
times for debugging various things that I think it makes sense to be a
proper debug macro.