[clang] [Clang] Extract offloading code from static libs with 'offload-arch=' (PR #147823)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> > but we now assume that if the user specified --offload-arch= on the link job, they definitely want that architecture to be used if it exists.
> 
> That would be my assumption, too. Do we currently just ignore `--offload-arch=` for the linking phase?
> 
> With the patch, what's expected to happen when link-time `--offload-arch=` specifies an architecture that the libraries were not built for and therefore have no corresponding blobs to extract. I assume it would be diagnosed. If that's the case, we may want to add a test case for that.

Right now we infer the ones to use from the contents of the `.o` files. This is more of a hint I guess? It's not a fatal error but I suppose it could be warning.

https://github.com/llvm/llvm-project/pull/147823