[clang] [clang][Driver][HIP] Add support for mixing AMDGCNSPIRV & concrete `offload-arch`s. (PR #113509)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> We don't use `offload` at the moment, that's for HIPSPV. Of course, future, fancy work is more than welcome, but this merely slots into the existing infra and current use cases.

I'm not a huge fan of smuggling what is essentially a Toolchain behind what was intended as a `-mcpu` option. I guess we already somewhat support that? I know @yaxunl was the one that made the original `--offload=` RFC / support but I don't think it ever got 100% finished. I'm just a little concerned that if we support it this way then we'll never be able to change it because the argument will be that we need it for backwards compatibility. But I guess that's already the case and this patch just simplifies it?

https://github.com/llvm/llvm-project/pull/113509