[llvm-branch-commits] [CI] Make premerge_advisor_explain write comments (PR #166605)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/166605

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.