GSym aggregated output to JSON file (#81763)

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

I've added the same capability to `llvm-dwarfdump` in a [different
PR.](https://github.com/llvm/llvm-project/pull/81762

)

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>