This PR adds full sycl-post-link functionality to clang-sycl-linker with 100% feature parity to clang-linker-wrapper, advancing the new SYCL offload model migration.

Changes:

• Implemented 4 core functions: getSYCLPostLinkSettings(), getTripleBasedSYCLPostLinkOpts(), runSYCLPostLinkTool(), and runSYCLPostLinkLibrary()
• Added 14 command-line options for controlling module splitting, ESIMD handling, specialization constants, entry point optimization, thin LTO, and device libraries
• Supports both tool mode (external sycl-post-link executable) and library mode (direct API) with configurable module split modes (kernel, source, auto, none)
• Triple-specific configurations for Intel GPU, Intel CPU, NVIDIA, AMD, and NativeCPU targets
• Added 32 comprehensive test cases covering all options, functions, and execution paths
• All 131 SYCL driver tests pass with no regressions

Note: sycl-post-link logic remains in clang-linker-wrapper for now during the offload model migration.