[llvm] [NewPM][BPF] Add `BPFCodeGenPassBuilder` (PR #94158)

[via llvm-commits]

4ast wrote:

> 
> this PR is just boilerplate and doesn't actually do much. however, it makes follow-up changes smaller which is valuable for reviewing

Not really. At present this is pointless code churn and nothing else. Make a meaningful diff and describe the reasons to make it.

https://github.com/llvm/llvm-project/pull/94158