[vishalshetagar]

-fsycl-link emits SYCL runtime registration references even when there are no device images to register. This prevents host-only and kernel-free configurations from linking with a host linker unless they also link the SYCL runtime.

This change makes both the old and new offload driver paths produce the required output object without references to __sycl_register_lib or __sycl_unregister_lib when there are no device images or all images are empty.

• Skip registration for empty image sets in both offload wrappers.
• Omit empty device modules from post-link image tables in both the tool and library implementations.
• Produce an empty object when the new driver's device pre-link step has no device inputs.
• Preserve registration for nonempty images, mixed empty/nonempty inputs, and NativeCPU placeholders.
• Preserve the required non-RDC container after empty-image filtering in the upstream flow that now performs wrapping during host compilation. Add an empty placeholder only when the combined image set is empty.

Extend existing regression tests for empty inputs, mixed targets, ELF/COFF wrapping, and host linking without the SYCL runtime. Fixtures whose section, registration, or missing-ocloc checks require a nonempty image use real kernels with integration-header inclusion disabled.

Validation after merging upstream 55a172cc383c:

• 139 focused regression tests passed: seven relevant Clang tests and the complete llvm/test/tools/sycl-post-link suite. Clang and the affected tools were rebuilt in WSL with assertions enabled; auxiliary tools and device libraries came from the matching upstream CI toolchain.
• Both driver paths passed Linux executable and shared-library linking without the SYCL runtime for host-only and kernel-free pre-link inputs. Executables ran successfully. Empty non-RDC inputs also passed; nonempty kernel inputs retained registration references.
• All 12 additional shared-wrapper checks passed across ELF and COFF, including mixed images and NativeCPU placeholders.
• Eight Windows executable cases linked with Microsoft's linker and ran; eight DLLs linked, loaded, and executed a test function without the SYCL runtime. Two nonempty-image controls correctly failed to link without the runtime. The empty non-RDC case used -nolibsycl.
• The new empty-container regression reproduced the missing-output failure before the follow-up fix and passed afterward. Formatting and whitespace checks passed.

Windows objects were generated by the rebuilt tools in WSL; linking and execution were native Windows. The compiler tools were not built natively on Windows.

The previously observed GPU E2E failures remain outside this fix:

• bindless_images/vulkan_interop/external_semaphore_regular_cl.cpp also failed in the base commit's CI job, before this PR's changes.
• The Windows WorkGroupMemory/basic_usage.cpp swap failure matches the existing reports in SYCL :: WorkGroupMemory/basic_usage.cpp has unrelated failures on Intel Arc With Level 0 Windows #22293 and [SYCL][E2E] Failures in unrelated tests on Intel Arc GPUs (Level Zero) with parallel E2E runs on Linux (B580) and Windows (A770) #23521.

These hardware-dependent failures were not changed or validated locally on Intel GPU hardware.

Fixes #23528.

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[sarnex]

@vishalshetagar Looks like the PR needs to be merged with HEAD since there are merge conflicts, could you take a look? Thanks

[vishalshetagar]

Thanks @sarnex I’ve merged the latest upstream sycl branch and resolved the conflicts. Retesting caught an empty-image issue with the updated non-RDC flow, which I fixed in the follow-up commit.

All 139 focused regression tests passed, along with Linux and native Windows linking checks. The changes are pushed and the PR description is updated.