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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

> > I don't understand why you check in auto-generated code. It is a common anti-pattern. You should instead make tblgen a build dependency. It is the common approach in the rest of the project. If you run e.g. `ninja llc`, there will be lot of tblgen invocations if the output was not generated in a previous run.
> 
> The main concern here is just that it's really difficult to review changes since we don't see the header (Since this is an API header the user will read and not just some implementation detail). I can see it either way honestly, it's just tough to review without seeing the changes.

Users will do the same thing as for llvm: `ninja install-offload-headers install-offload-libraries`.

When I invoked `ninja install-llvm-headers`, it first started to build support. The installed llvm headers depend on tblgen.

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