Add required field to CodeClimate export for GitLab code quality job

[jmalmari]

CodeClimate export of CodeChecker parse (and maybe others) doesn't produce as complete JSON as it could. Its array elements are missing a severity field which makes them not load in GitLab code quality widget. As severity information is readily available it would be quite easy to add it.

A GitLab CI job might do the following:

1. CodeChecker analyze -o ./reports compile_commands.json
2. CodeChecker parse ./reports -e codeclimate >codeclimate.json

The output doesn't qualify as a proper GitLab code quality report. The reason is that in GitLab the severity field is required whereas CodeClimate specification declares it optional. Relevant documentation:

• https://github.com/codeclimate/platform/blob/master/spec/analyzers/SPEC.md#data-types
• https://docs.gitlab.com/ee/user/project/merge_requests/code_quality.html#implementing-a-custom-tool

Example diff of the wanted JSON file:

[
 {
    "type": "issue",
    "check_name": "deadcode.DeadStores",
    "description": "Value stored to 'buffer' is never read",
    "categories": [ "Bug Risk" ],
    "fingerprint": "1421455f5cc176089ded56749a5d6535",
+    "severity": "minor",
    "location": { "path": "foo.c",  "lines": { "begin": 32 } }
  }
]

I made a proof of concept that works for my purposes: jmalmari/codechecker@ca9f9339

[csordasmarton]

@jmalmari Can you please create a pull request from your branch to our repository? So we can review it more easily and we can merge it inside our repository 😊