Move SYCL device-side macro initialization from Driver into FE

[AGindinson]

Generally, SYCL device-side macros are being initialized within clang/Frontend/InitPreprocessor.cpp, e.g. SYCL_EXTERNAL (https://github.com/intel/llvm/blob/sycl/clang/lib/Frontend/InitPreprocessor.cpp#L1180), __SYCL_DEVICE_ONLY__ (https://github.com/intel/llvm/blob/sycl/clang/lib/Frontend/InitPreprocessor.cpp#L1179), target-specific __SYCL_NVPTX__ (https://github.com/intel/llvm/blob/sycl/clang/lib/Frontend/InitPreprocessor.cpp#L1183). However, there are known inconsistencies with this approach:

1. FPGA-specific __SYCL_ENABLE_USM_ADDR_SPACES__ gets initialized within the Driver: https://github.com/intel/llvm/blob/sycl/clang/lib/Driver/ToolChains/Clang.cpp#L4683
2. __SYCL_NVPTX__ correctly gets initialized within InitPreprocessor.cpp, but is not LIT-tested within the corresponding sycl-macro.cpp.

Items TBD:

• Identify and move all device target-specific macro initializations into FE
• LIT-test these initializations alongside "general" SYCL device macros

[AGindinson]

__SYCL_NVPTX__ patch: #3977