[SYCL] Avoid runtime registration for empty device pre-link outputs

[vishalshetagar]

-fsycl-link currently 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, including mixed empty/nonempty inputs, and NativeCPU placeholders.

Extend existing regression tests to cover these cases and host linking without the SYCL runtime.

Validation:

• 134 tests passed: the two affected Clang tests and the complete llvm/test/tools/sycl-post-link suite, using rebuilt affected tools with assertions enabled and a prebuilt DPC++ frontend.
• Both driver paths passed Linux executable and shared-library linking without the SYCL runtime for host-only and kernel-free inputs; executables ran successfully. Nonempty kernel inputs retained registration references.
• 12 additional wrapper checks passed across ELF and COFF targets.
• Seven Windows executable cases linked with Microsoft's linker and ran; seven DLLs linked, loaded, and executed a test function without the SYCL runtime. Two nonempty-image controls correctly failed to link without the runtime. Windows objects were generated by the patched tools in WSL; linking and execution were native Windows. The compiler tools were not built natively on Windows.

The non-RDC wrapper and linker-wrapper command tests now use real kernels where they require a nonempty image. This preserves their section, registration, and missing-ocloc checks after empty images are omitted. Integration-header inclusion is disabled in these standalone fixtures, and the non-RDC test additionally checks empty ELF and COFF objects.

Follow-up validation with the PR's CI-built Clang 24 toolchain: both original compiler-test failures were reproduced, then passed after the fixture changes. The wider SYCL driver, linker-wrapper, and complete post-link selection finished with 294 passed, 3 expected failures, 15 unsupported, and no unexpected failures. An empty non-RDC COFF object also passed native Microsoft executable linking/execution and DLL linking/loading/calling with automatic SYCL library linkage disabled (-nolibsycl).

The GPU E2E failures are separate from these compiler-test fixes:

• bindless_images/vulkan_interop/external_semaphore_regular_cl.cpp reports "The external semaphore wait did not submit the preceding batch." The same failure is present 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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