[clang] [llvm] [Offload][NFC] Remove `omp_` prefix from offloading entries (PR #88071)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> How about `llvm.offload`? This might need broader discussion but I'm fine either way.

I don't think it's too important here since these are effectively internal variables. However in the future I want to combine `cuda_offload_entires` and friends into just `llvm_offload_entries`.

https://github.com/llvm/llvm-project/pull/88071