[llvm] [offload][SYCL] Add SYCL Module splitting. (PR #131347)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> Yes, a user chooses a target like:
> 
> ```
> clang++ -fsycl -fsycl-targets=nvidia_gpu_sm_90  code.cpp
> or
> clang++ -fsycl -fsycl-targets=amd_gpu_gfx1201 code.cpp
> ```
> 
> The list of supported targets in downstream can be observed here: https://intel.github.io/llvm/UsersManual.html#generic-options

This syntax unnerves me. You can kind of do this in OpenMP but the syntax is really verbose.
```
clang++ -fopenmp -fopenmp-targets=amdgcn-amd-amdhsa,x86_64-unknown-linux-gnu -Xopenmp-target=x86_64-unknown-linux-gnu --offload-arch=znver5 -Xopenmp-target=amdgcn-amd-amdhsa --offload-arch=gfx940
```
I want to let `-Xarch_` handle this but that requires changing a flag which had some detractors. I also have https://github.com/llvm/llvm-project/pull/125556 to make that generic but it needs to be updated.

https://github.com/llvm/llvm-project/pull/131347