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

[Konrad Kleine via cfe-commits]

kwk wrote:

Thank you @frasercrmck for the feedback. I see what I can do. For now I just want to add two massif profiles that I created:

[massif.out.zip](https://github.com/user-attachments/files/23751791/massif.out.zip)

Here's one visualized:

<img width="1870" height="1042" alt="image" src="https://github.com/user-attachments/assets/d03a2f0d-42f1-4f3d-b3d7-1259f08de034" />

> From a quick run through massif, it looks like MachineInstrs in SPIRVGlobalRegistry are the memory hog?

@nikic, unfortunately I couldn't verify this with my abilities. Both profiles seem to have no notion of `SPIRVGlobalRegistry`. Maybe this is because I don't have debug symbols.

```console
$ ms_print massif.out.15 | grep SPIRVGlobalRegistry
$ ms_print massif.out.17 | grep SPIRVGlobalRegistry
```


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