[clang] [clang][Driver][SPIR-V] Allow linking IR using llvm-link (PR #169572)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

I wish SPIR-V had a proper linker we could use. I wonder what would happen if you fed it through `ld.lld` since we're not using the backend or ELF at all, would probably be a weird use-case. I'm mostly concerned because this kind of deviates from the normal compiler flow. I wonder if @MaskRay has any good ideas here. 

https://github.com/llvm/llvm-project/pull/169572