[libclc] [libclc] Add an option to build SPIR-V targets with the LLVM backend (PR #151347)

[Nikita Popov via cfe-commits]

nikic wrote:

I don't think it makes sense to backport this. At least at the time it landed it only worked in the sense of "does not crash during compilation". I'm not sure if the big compile-time and memory usage issue has been resolved since, or anyone has checked whether the produced SPIRV actually works.

https://github.com/llvm/llvm-project/pull/151347