fix(check): correct diagnostic count

[a-frantz]

Describe the problem or feature in addition to a link to the issues.

Fixes diagnostic counting that I messed up in #59

Without this fix, "suppressed" Notes and Warnings will be counted and can lead to a confusing exit message.

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[a-frantz]

@peterhuene I goofed up in the last few PRs forgetting to update the CHANGELOG. I can cherrypick the commit I made on this branch into a separate PR if we want? Not sure how strict we want to be about the git history.

[peterhuene]

@a-frantz I think it's fine to fix it as part of this PR.