[clang] [Offloading][NFC] Move creation of offloading entries from OpenMP (PR #70116)

[Yaxun Liu via cfe-commits]

yxsamliu wrote:

Maybe a clang documentation can be added for this generic offloading toolchain for OpenMP/CUDA/HIP. Could be a brief description about the offloading entries and registration mechanism in the beginning then buffy it up later.

https://github.com/llvm/llvm-project/pull/70116