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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> In our specific case we don't need to run optimizations on the `llvm-link` output immediately as it will be optimized when it's used in a larger compilation command, but I can see the use case in general.

Yeah, we do these tricks in several places. Like `libc`, `libclc`, OpenMP, etc. It would be nice to have a way to remove the special handling required to make a single optimized `.bc` file in CMake.

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