[SYCL][thinLTO] Split early when thinLTO is enabled

[sarnex]

This change implements driver changes to call sycl-post-link during -c when thinLTO is enabled, and updates clang-offload-packager to accept response files (same format as temp file list), which will be generated when we call clang on each output of sycl-post-link.

I will add a test for the clang-offload-packager changes later, because we rely on changes in other PRs to test the full flow. However, the code is already being hit because we compile libdevice with thinLTO when compiling for the new offload model.

For a given file a.cpp that results in two total splits, the following will happen:

1. We call sycl-post-link on a.bc
2. We extract the "Code" column from the table output of sycl-post-link
3. We call llvm-foreach with the regular backend job that would be called normally on each bc file from 2)
4. We call clang-offload-packager with the output of 3 which is a list of the bc files from each clang run, with a '@' appended to the file path
5. clang-offload-packager reads each entry inside 4) and adds each to the fat object
6. clang-linker-wrapper extracts the fat object from 5) and finds all of the files
7. ... to be implemented

[asudarsa]

Hi @sarnex

Thanks for adding this change. Will it be possible to provide a short write-up that showcases a small example and show how the compilation proceeds with the changes here?

Thanks

[sarnex]

@asudarsa just added this to the commit description, thanks!

[asudarsa]

All changes outside 'Driver.cpp' looks good to me. Will let @mdtoguchi comment on that part.

Thanks Nick.

[mdtoguchi]

OK for driver. Even with the simplistic nature of what is being performed, all of the additional actions required to work the table/listfile isn't very pretty.

[sarnex]

@mdtoguchi Thanks Mike. If you have an idea on how I could rework this change to be cleaner please let me know, I'm happy to do it in a follow-up PR.

[sarnex]

HIP failure #14300