[llvm] [DeveloperPolicy] Add guidelines for adding/enabling passes (PR #158591)

[Sjoerd Meijer via llvm-commits]

https://github.com/sjoerdmeijer approved this pull request.

Looks like a good start to me, capturing the status quo pretty well. And we could iterate on it if we wanted to. 

Maybe give it a day before merging this in case someone disagrees or would like to leave more comments. 

https://github.com/llvm/llvm-project/pull/158591