[New offloading model][SPIR-V backend] Add SPIR-V backend support inside clang-sycl-linker

[asudarsa]

For
clang --sycl-link --sycl-use-spirv-backend ....
We invoke clang-sycl-linker like this
clang-sycl-linker -use-spirv-backend .....

This causes the invocation of the SPIR-V backend to translate LLVM to SPIR-V (instead of the llvm-spirv translator).

Following is a list of changes in this PR:

1. Add -sycl-use-spirv-backend to list of clang options
2. Add -use-spirv-backend to list of clang-sycl-linker options
3. Enable SPIR-V backend in the SYCL compiler build (add it to LLVM_TARGETS_TO_BUILD)
4. Enable building of clang-sycl-linker in the SYCL compiler build (add it to sycl/CMakeLists.txt)
5. Call SPIRVTranslate API call inside the clang-sycl-linker when appropriate options are specified

Please note that this is a work in progress. Options specified to the SPIR-V backend will be improved in near future.

Thanks

[bader]

1. Add -sycl-use-spirv-backend to list of clang options

Can we use -fsycl-targets=spirv64 instead?

• Add -use-spirv-backend to list of clang-sycl-linker options
• Call SPIRVTranslate API call inside the clang-sycl-linker when appropriate options are specified

I suggest we make clang-sycl-linker tool changes in the upstream first.
NOTE: Upstream uses SPIR-V backend by default already: llvm/llvm-project#129545, so the patch for the upstream should be smaller.

[asudarsa]

I had an offline discussion with @VyacheslavLevytskyy and we agree with @bader's comment. I will clean this PR up to remove the translator (and other unneeded options) and use SPIR-V backend by default inside clang-sycl-linker. I will mark this as draft for internal review. Once we have consensus, I will submit this upstream.

On second thoughts, I think we can close this PR, and I will put up a draft PR upstream for internal comments.

Thanks all.

[asudarsa]

I will be opening an upstream PR soon. Closing this. Thanks for the feedback here.