[llvm] [Offload] Implement `olShutDown` and remove global reference counting (PR #144055)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

Initialization like this should *always* be reference counted. Otherwise the first thread to call `olShutDown` will destroy the context for everyone.

https://github.com/llvm/llvm-project/pull/144055