Feature: prevent a formula’s previous version from being removed by brew cleanup

[bfontaine]

This is inspired from this question on StackOverflow. OP wants to keep gcc 4.9 and 5.2, but running brew cleanup would remove the former.

I don’t know if this is something that would be used, or another niche feature that only a few people would use. It shouldn’t be hard to add something in a formula’s prefix to mark it as non-cleanable.

[tdsmith]

The versions formula seems like a better solution.

[DomT4]

My answer in #42036 applies here a lot as well.

[tdsmith]

I think my biggest concern with this is that it's hard to restore this state where both versions are installed and switch can be used to switch between them. I think switch is a good backstop to have in case a new release breaks something but I don't think we should be encouraging people to make it part of their workflows.

[MikeMcQuaid]

Agreed with @tdsmith here. It'd be good to know how/if we can improve the versions tap to make these workflows better.

[bfontaine]

Thank you all for your feedback, I’m closing this.