[CI] Make premerge_advisor_explain write comments
This patch makes the premerge advisor write out comments. This allows
for surfacing the findings of the advisor in a user-visible manner
beyond just dumping the output in the logs. Surfacing the information in
a comment also makes it much easier to discover compared to the Github
summary view.
Reviewers: dschuff, gburgessiv, Keenuts, DavidSpickett, lnihlen
Reviewed By: DavidSpickett
Pull Request: https://github.com/llvm/llvm-project/pull/166605
