[llvm] [Offload] Introduce offload-tblgen and initial new API implementation (PR #108413)

[Callum Fare via llvm-commits]

callumfare wrote:

Thanks @jhuber6, I have it all working now. The depfile issue was some weird behavior with relative paths so I just moved where the relevant cmake around a bit.

In summary:
* The generated files are all checked in and formatted, no tablegen is required to build the library.
* If someone changes the API, they need to build the `OffloadGenerate` target to regenerate the files which are all copied back to the in-tree directory. If clang-format isn't found, the target won't be available.

https://github.com/llvm/llvm-project/pull/108413