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

[Joseph Huber via cfe-commits]

jhuber6 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.

We have https://clang.llvm.org/docs/OffloadingDesign.html which describes the entries, but from an OpenMP perspective. But I have thought about reworking it once I finish up the new driver support (which this is a part of).

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