[llvm] WIP: [Offload] Add testing for Offload program and kernel related entry points (PR #127803)

[Callum Fare via llvm-commits]

callumfare wrote:

@jhuber6 I've addressed some of your comments on here on the original PR (#122106). After fixing an issue with the __tgt_device_image lifetime I've realised I no longer need the plugin changes, so I'd like to roll these two PRs into one, since having reviews split across both is confusing. I'll close this PR soon and move the changes across.

https://github.com/llvm/llvm-project/pull/127803