[llvm-dwarfdump] --statistics: break lines and indent by 2

so that the user does not have to pipe the output to `jq` or `python -m json.tool`.
This change makes testing more convenient because `-NEXT` patterns can be used.
The "prettify by default" is a good tradeoff to make. The output size increases a bit.

Differential Revision: https://reviews.llvm.org/D86318