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

[Konrad Kleine via cfe-commits]

kwk wrote:

@nikic, this looks more like your observation I think... 

<img width="2486" height="1562" alt="image" src="https://github.com/user-attachments/assets/4de3d1b2-05c4-4ab1-be08-49dda1759210" />


[massif.out.zip](https://github.com/user-attachments/files/23886104/massif.out.zip)


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