Lint add warnings instead of check failure

[Snorch]

There are multiple cases where good human readable code block is converted to an unreadable mess by clang-format, so we don't want to rely on clang-format completely. Also there is no way, as far as I can see, to make clang-format only fix what we want it to fix without breaking something.

So let's just display hints inline where clang-format is unhappy. When reviewer sees such a warning it's a good sign that something is broken in codding style around this warning.

[Snorch]

It looks like that:

Please remove draft/test: introduce some codding style errors to test new approach when merging this PR, this commit is needed just to illustrate the concept.

[codecov-commenter]

Codecov Report

Patch coverage: 84.61% and project coverage change: +0.06% :tada:

Comparison is base (5fedcaa) 70.62% compared to head (6ce24a5) 70.68%. Report is 14 commits behind head on criu-dev.

:exclamation: Current head 6ce24a5 differs from pull request most recent head 99d878b. Consider uploading reports for the commit 99d878b to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## criu-dev #2250 +/- ## ============================================ + Coverage 70.62% 70.68% +0.06% ============================================ Files 133 133 Lines 33322 33328 +6 ============================================ + Hits 23532 23557 +25 + Misses 9790 9771 -19 ``` | [Files Changed](https://app.codecov.io/gh/checkpoint-restore/criu/pull/2250?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=checkpoint-restore) | Coverage Δ | | |---|---|---| | [criu/proc\_parse.c](https://app.codecov.io/gh/checkpoint-restore/criu/pull/2250?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=checkpoint-restore#diff-Y3JpdS9wcm9jX3BhcnNlLmM=) | `68.54% <50.00%> (-0.03%)` | :arrow_down: | | [criu/memfd.c](https://app.codecov.io/gh/checkpoint-restore/criu/pull/2250?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=checkpoint-restore#diff-Y3JpdS9tZW1mZC5j) | `82.77% <85.71%> (+1.00%)` | :arrow_up: | | [compel/src/lib/infect.c](https://app.codecov.io/gh/checkpoint-restore/criu/pull/2250?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=checkpoint-restore#diff-Y29tcGVsL3NyYy9saWIvaW5mZWN0LmM=) | `54.67% <100.00%> (+0.18%)` | :arrow_up: | | [criu/files-reg.c](https://app.codecov.io/gh/checkpoint-restore/criu/pull/2250?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=checkpoint-restore#diff-Y3JpdS9maWxlcy1yZWcuYw==) | `75.38% <100.00%> (+0.02%)` | :arrow_up: | ... and [5 files with indirect coverage changes](https://app.codecov.io/gh/checkpoint-restore/criu/pull/2250/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=checkpoint-restore)

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[Snorch]

I've added changes to CONTRIBUTING.md with bad clang-formating examples to PR and a link from warnings to CONTRIBUTING.md, so that it would be easier to understand how this warning should be handled.

[Snorch]

I've removed test commit to trigger warnings (it works), so it's now ready to merge.