llvm-dwarfdump --verify aggregated output to JSON file (#81762)

In order to make tooling around dwarf health easier, I've added an `--verify-json` option to `llvm-dwarfdump --verify` that will spit out error summary data with counts to a JSON file.

I've added the same capability to `llvm-gsymutil` in a [different PR.](https://github.com/llvm/llvm-project/pull/81763)

The format of the json is:
``` json
{ 
  "error-categories": { 
    "<first category description>": {"count": 1234},
    "<next category description>": {"count":4321}
  },
  "error-count": 5555
}
```
for a clean run:
``` json
{ 
  "error-categories": {},
  "error-count": 0
}
```

---------

Co-authored-by: Kevin Frei <freik@meta.com>

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