[llvm] [Offload] Move (most) global state to an `OffloadContext` struct (PR #144494)

[Ross Brunton via llvm-commits]

RossBrunton wrote:

I'm pretty sure I updated this, is this reproducible with a clean build?

@jhuber6 I can have a look on Monday. If you want to revert for now, feel free.

https://github.com/llvm/llvm-project/pull/144494