Upstream SPIR-V built-ins declarations

[Fznamznon]

We have to upstream SPIR-V buit-ins declarations (enabled by #1384 ) to llorg. Otherwise we can't compile any SYCL sample using our SYCL headers and llorg clang:

.../build/bin/../include/sycl/CL/__spirv/spirv_ops.hpp:26:2: error: "SPIR-V built-ins are not available. Please set -fdeclare-spirv-builtins flag."
#error                                                                         \
 ^

[bader]

We can put an explicit header with the built-in declarations as a mitigation measure under ifdef and replace error directive with a warning/note.